Genuine 3" Colt Pythons are very rare and now very valuable collectibles.
The first was the famed "California Combat" which was made for/by a big west coast distributor.
8" Pythons were cut to 3" because the vents in the barrel rib of the 8" models worked out right for a 3".
I seem to recall that the actual conversion was NOT done by Colt.
The genuine Colt factory 3" is not marked in any special way, and was only sold for a couple of years.
Problems is, after Colt shut down production, they sold all the left over 3" barrels to Gun Parts Corp.
Lots of people bought these 3" barrels and re-barreled standard Pythons.
There are so many of these re-barreled Pythons out there, any Python with a 3" barrel is suspect.
Not only are there fake 3" Pythons, there are also boxes with fake factory end labels AND fake "factory letters", listing them as genuine.
These days, due to the extreme value of a 3" Python, a smart buyer buys his OWN Colt factory letter before paying for the gun. A few too many people have waited until after buying the gun, then get a factory letter saying the gun was shipped with a different barrel length, or find out the "factory letter" is a counterfeit, just like the gun.
When we're talking this kind of money, trust no one.