I think you're correct (but it's 35mm). I think mentioning the film size was an oversight by the author, or maybe since the camera was from 1971 we were supposed to assume.
Apparently, if you hold this camera horizontally, it shoots a portrait orientation photo of the size you say whereas a "normal" 35mm camera would shoot a landscape orientation photo that is 36mm x 24mm.
But I did notice that the number 40 was red on the counter implying significance. I quick google search revealed the same question and answer plus additional manuafacturer: AGFA
https://www.photo.net/discuss/threads/20-exposure-35mm-film-...
"Got an offer to buy several hundred rolls of packaged Agfa APX 400 speed film. It is all in 20 exposure cassettes. Where have I been that I don't recall "20 exposure" rolls? Were they a special run for point-of- purchase at some kind of strange outfit like (insert schlock discount store here) or a serious product?...."
"They used to make 20 expsure rolls before they switched over to 24 per roll. That change happened in the 70s I believe, so the film is probably pretty old if it has got only 20 exposures per roll. While thats my best guess, you could be right, and it could have been special order."
The camera is from 1961 so would have reference to 20 exposure rolls.