Without going down the rabbit hole of complicated speculation, the simplest explanation is that the barrel was changed sometime after 1912, and re-proofed under the new nitro proof system. To factually determine this--or not--requires close hands-on examination.
There is not evidence that the adoption of the nitro proof was anything other than a hard changeover, which would not provide for a transition period with the possibility of mixing actual proof stamping.
--Dwight
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