From what I've read and come across with Pietta Replicas ... most do or could have authentic backgrounds. As Colt did only produce certian models on a regulaproduction line ... I've read where the barrel lengths could be specified to customer's request.
So I'd say you could have gotten this one from Colt on special order, reguardless of other non believer's tales
I have one and with a Kirst .38spl/.38Colt konvertor. This was and I'veread it existed for the 1862Y(year made) Army Police Revolver...I wished I'd saved the story on it.
Besides that many Post Civil War Revs were changed as they found their way into the Civilian sector along with special requests made by Soldiers during the War and adhered to by the Armor, i.e. 1851 Navy barrels shot out and rebored to a .44cal C&B and fitted with a n1860 Army Colt cylinder ... anything could have been achieved ...
But Colt Factory only offered particiular standard models with an optional Barrel length offer.
Hope that helps...it's my findings and beliefs anyway from what I have found to have existed.
Here's Mine a Convertable .36 C&B & .38SPL Pietta/Kirst Colt 1861 5.5"bbl
Colt Factory made this combination iof the 1849 Poket and the 1862 Pocket police in .38Colt(Long) so nothing would surprise me ...