1906 AE BUN proofed
 

That's correct, it's not a typo...
C/B C/U on left receiver and C/N on the barrel. The sereial# is at the beginning of the transition from BUG proofs to C/N proofs. What do you think? Comments please.

https://www.gunbroker.com/item/843150251

I have no affiliation with the seller.

It's an interesting piece. Somebody with more expertise than me might have insight.

Likewise, I have no affiliation with the seller.

To prompt discussion, it's worth posting photos:

I think it is just what you said, a pistol from the transition from BUG proofs to the C/N.
An interesting example in nice shape, but not worth the $2500. JMHO.

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