I think what is seen as a main spring screw adj hole in the front may just be where the wood was cut too thin at one time during a refit/refinishing job.
The area punched through and then was cleaned up a little to make it more presentable looking.
It still doesn't have that 'drilled' or cut out appearance. Kind of rough around the edges and quite low for an adj screw.
A clue to reworking the grips may be on the LH panel. Look at the checkering pattern in the extreme lower LH corner (pic#1).
That area was gone over to recut a few checkering lines and not with the same quality of skill as the rest of the work.
The wood likely (IMO) was shaven down in that area removing the checkering and then the lines recut along with their over runs.
That slimming of the wood may have included the front grip strap of the grips and done the thinning of the wood leading to the break-though appearing as a screw adj hole of sorts.
The grips inletting do have some appearance to fitting some of the early English revolvers, A Webley New Model Army No.5 is close, but the rounded top area is not quite right.
Maybe something like that or othr foreign made as far as the firearm they fit.