Very common:

30 (1976): You probably know this one. No doubt the best-selling calculator ever.

55 (1977): Budget scientific model.

59 (1977): Surprisingly common, given its expense and sophistication. Sometimes found with the PC-100 printer.

1025 (1977): Often found in the original box.

1200 (1975): Also often found in the original box. The 1200 series styling was used on many other TI calculators and they all shared the annoying characteristic of loose or missing faceplates.

1250 (1975)

1750 (1977): TI's 1st LCD calculator.

2550 (1974)

Business Analyst - I (1979)

Little Professor - 2 versions (1976, 1978): On the early version, the on/off and level switches are on the side; on the later version, they're on the front.

Common:

57 (1977)

58C (1979): Model 58 with continuous memory.

1000 (1977)

1270 (1976): Colorful model. TI appeared to like orange.

2500 Datamath - 2nd version (1973): There are two versions. The first earned a "best buy" from Consumer Reports; the second earned a retraction from Consumer Reports five months later. How can you tell the difference? The earlier has a recharging time of 16 hrs. (stated on the back of the calculator); the second recharges in 10 hrs. Apparently there are also differences in the display angle and brightness (which resulted in Consumer Reports' action).

2500 - II (1974): Similar to 2500 but has % key and cheap logo strip that tends to peel off. Eventually sold at give-away prices.

SR-10 - 2nd version (1973): Again, two versions. In the first version, the "SR-10" logo is situated in the escutcheon surrounding the display. In the 2nd version, the logo moves down to the upper area of the faceplate. All advertising photos, manuals, and boxes I've seen show the 1st version, though it's far less common.

SR-11 (1973)

SR-40 (1977)

SR-50 (1974)

SR-50A (1975)

SR-51-II (1977)

SR-52 (1975)

Business Analyst (1976): The first in a long line of popular financial calculators.

Moderate:

58 (1977): Replaced with the 58C in 1979.

1050 (1977): Another entry in the 1000/1025 line, but not nearly as plentiful.

1265 (1976): Has a blue fluorescent display, unique within the 1200 series.

1400 (1976): Has a brown carrying case, not the usual black. Similar styling to the 1200, but a little higher quality.

1450 (1976)

1500 (1974): Nifty little unit with thick chrome borders - one of my favorites.

1600 (1976): The 1600 series consisted of slimmed-down models that prefigured the styling of LCD units. They are surprisingly difficult to find.

1650 (1976)

1680 (1977): This one could re-display previous entries and allowed some primitive editing of those entries.

2500 Datamath - 1st version (1972): TI's first pocket calculator.

2550 - II (1975)

2550 - III (1976)

5050M (1976): Handheld printing unit with memory.

DataMan (1977): Very unusual child's calculator that looks like a robot.

MBA (1977)

Money Manager (1977)

Programmer (1978)

SR-10 - 1st version (1972): TI's first "SR" (Slide Rule) model.

SR-16 - II (1975)

SR-51A (1975)

SR-56 (1976)

Difficult or Very Difficult:

76 - "Spirit of 76" (1976): Red/white/blue Bicentennial (of the USA) model. Another 1200 progeny.

150 (1974): Possibly the rarest TI. The keys and body style of 2550, painted-on silver trim of SR-10. No memory or fixed decimal switch as on 2550. Unusual orange display.

1200 "Lady" (1975?): The 1200 with more "feminine"-colored faceplate and keys.

1205 (1976): Another colorful 1200 relation, but this one has oversized lettering too.

1220 (1976): This (and the 1225) are somewhat obscure, but otherwise ordinary, 1200 series units.

1225 (1976)

2510 (1973): Similar to 2500, but chronologically sandwiched somewhere between the

2500 and 2500 - II, probably failing to land a market niche in TI's crowded product line. Calculator body is a lighter cream shade compared to 2500's beige. Uses disposable batteries.

5050 (1975): I believe this was the 1st handheld printing calculator with vertical printing.

DataClip (1977): LCD calculator shaped like a 6-inch ruler. High-quality, unlike its many knockoffs.

SR-16 (1974): Superseded by the SR-16II within a year. This model is borderline moderate/difficult.

SR-51 (1975): TI replaced the SR-50 and SR-51 in mid-1975 with the "A" models. The SR-51 had been marketed only a few months when this happened, so it's quite rare. This calculator is also more stylish than any of TI's other advanced models. Another one of my favorites.

Wiz-A-Tron (1976?): Child's calculator similar to Little Professor but with wizard rather than owl on face. Very colorful.

TI made calculators for sale under other names, including Radio Shack. These are more difficult to locate than the TI versions. Some examples are (1) the Longines Symphonette 2500, a clone of the TI 2500 Datamath (2nd version), but in a green color, (2) the Exactra line (models 19, 20, 21, 22, and 23), reminiscent of the more colorful 1200 models, but sold as early as 1973, and (3) many Montgomery Ward models (identified by the "TXI-" prefix to the model number). The Montgomery Ward calculators are especially fascinating as they are not obvious clones yet can be identified as related to specific TI models.

Some models are difficult to find in good condition. The SR-10 and 11 are notorious; the silver trim is almost always worn on these models. An SR-10 in near mint condition would definitely qualify as "Difficult". The SR-50, 51, and 52 suffered from paint wear around the switches and later TI models often have loose nameplates and missing battery covers. Other models, such as the 2500 and 2550, are usually in quite nice condition. Most models are rarely found with the original box and paperwork; out of dozens of TI 30s that I have seen, only 1 or 2 have been new-in-box. Interestingly, some of the earlier models, such as the SR-10 and SR-50, appear to be easier to find with the box, although the boxes tend to show wear easily.

Since my article first was printed, I have discovered/been informed of additional models, which all appear to be somewhat rare. These include:

Concept 1 and 2: same style as 1200 series but with a two-tone rust and gray faceplate.

SR-22 (1973): desktop-sized model with programming functions such as hexadecimal and octal conversions.

2500 Datamath (LCD version): Prototypes were produced, but display unreliability prevented it from being offered to the public.

45: scientific model with a fluorescent display; similar in styling to the TI 30.

Longines-Symphonette Electronic Calculator (1972): Last issue, I mentioned a Longines-Symphonette similar to the TI 2500B. I have since discovered one similar to the early version of the 2500. This one has a serial number of only 3201 and the (TI) charger has a date stamp of July 1972, just one month after TI started production of the 2500, indicating that TI was manufacturing these clones right from the start. It has the same green case color as the other.

Finally, TI calculators often have a data code stamped on the back. It appears to consist of a two-digit week and two-digit year, such as "4376", for the 43rd week of 1976. TI chargers, too, have date codes, expressed as month and year, such as "7 72".