I poked around dictionary.com, wikipedia, wordnik,and various web sites about small mechanical calculators, but couldn’t find a totally satisfying answer. But the best fit, although still a highly speculative one, seems to be the “Addiator” calculator. This was a mechanical calculator that was very light for the time, but it would not literally fit in your pocket. And, strictly speaking, an Addiator salesman would be an anachronism, but far less of an anachronism than any calculator that would truly be pocket-sized.
Interestingly enough, True Grit was WRITTEN before true pocket calculators, as we know them, were widely available on the market. It was written in 1968. The first electronic calculators, which were also the first true “pocket calculators”, began being sold in 1969 and didn’t really take off until the 1970s, and even then were prohibitively expensive for most folks.
One possibility that occurred to me was that the “pocket calculators” were slide rules. Slide rules wouldn’t literally fit in your pocket but were far smaller than any mechanical calculator at the time, so if pocket calculator was a slang term for slide rule, it would be an exaggeration but not an absurd one. And slide rules existed in the mid to late 19th century (well before that, in fact) but I can find no evidence that a slide rule was ever called a “pocket calculator”. Small slide rules are sometimes called “pocket rules”, but “calculator” seems to have been used only for mechanical adding devices, and not for slide rules of any size.
There were, generally speaking, four different types of “mechanical” calculators that were made in the mid 19th to early 20th century: the arithmometer, the comptometer, the “millionaire” calculator (turn of the century), and pinwheel calculators. But all of them were too big to possibly go by the name “pocket calculator”, unless the term was being used as a joke. And, again, I can find no evidence these devices were ever called, jokingly or otherwise, pocket calculators.
The Curta calculator was a true, hand-held, but MECHANICAL, calculator that could lit in your pocket. So, it might seem to be a good choice. But it was developed in 1947. While a certain amount of anachronicity is forgivable, that seems like too big of a stretch. And, I can find no evidence that it was called a pocket calculator at the time. Rather, the label seems to have been applied to it by historians after the electronic hand held calculator was invented.
The Addiator was sold in the 1920s and onwards. The term was applied to both “genuine” Addiators and clones made by competing companies: much like Xerox, the brand became synonymous with that general type of machine. It was MUCH smaller than any mechanical calculator available at the time. It wouldn’t literally fit in your pocket, though. While the 1920s would still be too recent for it to be historically accurate to have an Addiator salesman prancing around post civil war Oklahoma, it still seems plausible that a writer in the late 1960s, who is setting a story at a somewhat vague point in the late 19th century, would feel he could get away with a mild stretch of history like this.
I became very enamored with my Addiator theory when I found a vintage photo of an Addiator model with the words “Kingson Pocket Calculator” printed on said Addiator. While this doesn’t prove that “pocket calculator” was a general slang term for the Addiator class of calculators (it could have just been a brand slogan for the Kingson), it is at least a link of some kind between those calculators and that term, which is more than I can say for any of the other mechanical calculators. But, unfortunately, when it looked into it more closely, I saw that the Kingson model was developed in the early 1960s, which is much too late in time for it to lend any real support to my theory. It is possible that the phrase “pocket calculator” was applied to the earlier Addiator models sold in the 20s and 30s, but that is pure speculation, and the earlier models were bigger than the Kingson. Also, when I looked at the advertisement details for the Kingson, the advertiser expressly noted that it was the size of a “pocket comb”, suggesting that the descriptor “pocket calculator” was applied to the 1960 model in particular because it would (at least sort of) fit in your pocket.
http://www.vintagecalculators.com/html/addiator.html (scroll down a bit to find the Kingson).
Sigh. Its always a shame when a perfectly good theory founders on the rocks of inconvenient facts.
Despite this setback for my theory, the Addiator calculator seems as good of a possibility as any for what the writer of True Grit might have had in mind, and, by process of elimination, less implausible than the alternatives: slide rules don’t seem to have ever been called calculators, and every other mechanical calculator available anywhere near the right time period was much too big for the term pocket calculator to be remotely apt.
I’d be curious if anybody else, with access to better historical references than I used, could come up with a better explanation.