IIRC, I mentioned non-metallic cartridges as a very real possibility, except they didn't come in boxes, but in packets like those shown. I repeat, fiction stories are fiction, not tech manuals. The writer could as easily have written ".33 caliber", or ".48 caliber", or ".29 caliber" or, better, just said "a box of cartridges." Writers who are without technical knowledge should stick to generalites.
Everyone at Colt knew the Thuer conversion was a stop-gap until the Rollin White patent expired, but Colt really did work on it. They converted pocket models (1849, 1862 Police, and 1862 Pocket Navy), Navy models (1851 and 1861) and the 1860 Army. They also converted sidehammer revolvers and sidehammer rifles. Ammunition was made in .31, .36, and .44 calibers and sold in packets like the percussion cartridges.
But only some 3000 Thuer conversions were made in total, and they were uncommon. Always scarce, after 1873 the ammunition became almost unobtainable, which is why I said that no one would be buying a box of Thuer cartridges. It would be a bit like writing a story today in which the hero goes into the local store and buys a box of trounds for his Dardick pistol.
Jim