It certainly appears you were correct in identifying the proofs as 1887-1896 marks Both the 12/C in diamond & the word Choke rather than not for ball i9ndicate that. Right bore was proofed @ 13 (0.710"<0.719") & left bore @ 13/1 (0.719"<0.729"). Realize as I stated earlier these are not exact dimensions but the "Range" they fall in. It would be quite conceivable that the right bore was 0.718" & the left bore 0.720" which would be compatible with the marks but in fact, there would only be 0.002" difference between the two bores.