Just in case you have an old (pre-WWII) gun, check to see if the upper end of the backstrap has a checkered oval or lines running across. If it has the oval, it is for standard velocity .22 LR only, not high speed or modern hot loadings. (The top of the backstrap is the stop for the slide.)
At one time, Colt would sell new type backstraps, but those days are long gone.
Jim