Most 1900s I've seen have pellet counts on the barrel lugs, but I have #3656xx that is marked MOD on the right and 27 (probably meaning 327) on the left.
Charles Semmer's book on Remington doubles has a chapter on the pellet counts and other codes and marks stamped on the guns. Except for the pellet counts, the meaning of many of the other numbers, letters and codes are not known for sure. Mr. Semmer didn't mention there being choke markings other than pellet counts, but we now have evidence that at least a few are marked verbally.