Actually, the missing screw retainer for a Parker vise is one of the easiest parts to re-make.
Simply make a pattern by 'whittling' the part from wood, checking for fit, then get a little bit of fibre-glass cloth and resin, build up the the pattern a bit to compensate for 'shrink', then, after sanding the pattern smooth, take the pattern to a non-ferrous foundry and have it cast in bronze.
File to fit as needed, drill and counterbore for the screw, and....as the old saying goes.....'Bob's your uncle'....... : )
(of course, having a CNC mill is a much more 'elegant' way to make a 'really nice' replacement part, to be sure.)
cheers
Carla