I have a GH 16 circa 1900 with a plain grip cap. Not a screw touched on this gun so I doubt anything stock wise was done to it. I have few pistol grip guns for comparison but the ones I have all say Parker Brothers on the cap, except this one.

The plain grip cap with the oval checkered pattern in it was used into the early to mid-990's but it's not impossible that Parker Bros. used it on your GH 16.