The stamp does not appear to be a professional stamp.
Pros usually have a special stamp made for them.
This stamp is obviously a hand stamp done with individual letter stamps. Few pros would use this marking method.
However, considering the time period anything is possible.
Usually the only marking you see inside Colt factory grips is a hand written serial number in pen or pencil that was used to make sure the fitted grip went back on the correct gun during final assembly.
My best guess is the stamp was done by an owner.