Many years ago Gordon Herigstad, to whom I supplied many s/ns of imported Colts, visited me. I showed him the original horizontal grip that came on my overstamp Navy from the Erie County NY Sheriff's Dept., c.1974, which I had removed and replaced with an original '21 vertical foregrip for shooting. This horizontal is 8-1/8" long, 1/4" shorter than the WW2 version, and has the Remington offset swivel mounted on the underside. Gordon wanted to borrow it to copy, so I loaned it to him, and he returned it a few months later. I never bothered to ask him whether he simply had taken measurements for a pattern, or actually had copies made, so there may be some repros out there.
The finger grooves, incidentally, are 0.55" wide, and 5.50" long on the left side, 5.40" on the right.
M