any of the reproofs, up until 1954, could have been the origin of the left tube receiving the "choke" stamp....at some point, any constriction of 8 thou (or more) earned a "choke" stamp - and as parabola points out that condition might have been met many decades after the gun was built.
on diggory's handy chart of english proof marks, it becomes apparent that it is possible to see a gun built - without chokes, that might possibly go through proofing with exactly the same markings from 1855 until 1887. in the 1875 - 1887 era, he shows the london markings as "12 or 12B 14M"....while the brummie markings show only the "12B 14M" (but, that might just be for lack of space in his format....why would a "B" be needed if there were no choke?).
my dougall lockfast appears to be very close to the 1875 dividing line (sn 3147 - with london proofs), and i have puzzled if there might be a way to definitively place it "before/after"....but it was built cyl/cyl - and the proofs offer no indication.
looks as if they could have shown a bit more foresight, in differentiating between the time periods....but, as i often say - none of us need glasses for hindsight.
best regards,
tom (and, i am reassured to know that i'm not the only fellow spending time here over the Christmas weekend - hope your's is happy & bright)
