Python with off-center rollmarking.
I do not own a Python, but maybe when I go back to Mexico I'll get myself a 2.5 incher done up as a Diamondback. I posted a blurb about a photo that appeared in a thread in the Pistols and Revolvers area titled "Best looking handgun ever made" and got a PM asking me if I would copy-and-post my reply here in this thread. I hope this works.
Not to deviate from the thread, but the barrel markings on that particular model Python remind me of the early days in Mexico when we were buying 8 inch Target Python barrels rollmarked as .38 Special and installing them onto .357 Pythons so we could register them. Obviously, nobody wanted an 8 inch barrel on their Python and everyone wanted either a 6 inch or 4 inch model. 6 inchers were easy to make as they just cut off the back 2 inches of the barrel, turned it on a lathe and threaded the pipe that fits into the frame and "Bob was your Uncle". It looked perfect except that the roll-marking was offset towards the backside, just like in your photo.
Only a few years later, it became possible for us to fill in old rollmarking with molten metal, polish it round and then re-engrave using proper fonts ANY type of markings and reblue the result so perfectly that even those of us involved in the efforts could not distinguish a remark from an original. And obviously, neither could the Mexican Military. A large number of unregisterable .357 revolvers were thus remarked as .38 Specials and registered without problems. The process now, evolved over years of experimentations by the Master Gunsmiths working on the problem has become true artistry.
This 2.5 inch Python -- remarked in the proper fonts and style of Colt Firearms to appear to be a 2.5 inch Diamondback -- serves as an example. Now-a-days no self-respecting gun-nut in Mexico would cut-down an 8-inch Target Python barrel, nor even waste the time and effort to buy one from Numrich and smuggle it down. Any Python that needed to be registered would be made-up to be a Diamondback and registered as such straightaway. .38 Special Python cylinders are available from the Custom Shop for loan to be used at "time of registry" in case an over-ambicous Officer decides to just check and see if a .357 round will fit and allow the cylinder to close before signing off on the registry. As Jeff Cooper said: "Silly regulations provoke transparent evasions". But your photo of that Python with those off-center markings brought all that back.