I believe the OP is asking about these little gems:
They are rarer than hens teeth now, I found mine many years ago at a now out of business website called Speed Shooter Specialties that had some Performance Center parts.... in retrospect, I wish I had also bought one of the Performance Center Briley barrel bushings they had.
As you can see in the bottom pic, the frame on a PC pistol has a wider opening so there is a space for the legs on the magwell to straddle the grip. The pin that retains the grip also retains the magwell, which has to be drilled for the pin.