what tells you it is not original just a question I know very little about colts this is my grandfathers gun he bought it for my grandmother at an army base before he went over to WWII from a army captain it has K.J.B. stamped in bottom of the butt my father now owns it and colt wants a good bit of money to trace it plus they still have his AR-15 that they have had for several months so he is not to impressed with them right now but they are fixing it free of charge.
Here's some ways to tell it's not a factory Fitz gun:
The barrel is a cut down with a home-made front sight.
Note how the lettering is so close to the muzzle. That's because stamped in front of that was COLT in larger letters, and Police Positive in smaller letters.
Note the square butt grips and the longer ejector rod. This is a good indication that the gun was actually a Colt Police Positive Special, NOT a Detective Special.
Note that there is no Colt "verified proof" mark on the rear of the trigger guard.
The Colt "VP in a triangle" is stamped on the FRONT side of Colt trigger guards.
When genuine Fitz Colt's were made, the "VP" was stamped on the REAR side because the area at the front was cut away on the Fitz.
No "VP" on the rear means it was not a factory gun.
The way in which the hammer spur has been cut off is wrong.
A genuine factory Fitz would also have high quality checkering on the top.
In other words, this was a Colt Police Positive Special with a 4" inch or 6" inch barrel that was cut off, a home made front sight was attached, the trigger guard was cut off, and the hammer spur was removed.
Genuine Fitzgerald Colt's are VERY rare, these homemade Fitz copies are quite common.