Hello David
Looked more closely, there is no common denominator, the PSU cases are interchangeable so who's too know the original configuration unless it's NOS still in the box.
The PSU cases I have come in two types with various green crackle, green paint and brown paint, on one type the key holes on top are closed by an interior plate with the side key holes being an added plate so no through hole in the actual case. The other type has through holes on top and the sides, requiring a grommet.
Closed holes are probably later; as we had more steel than rubber.
In the EMER 256/3 it lists a grommet but I think it is for the small round holes for mounting the Variometer Packing Piece.
EMER 256/3 WS19 Mk.III pg.60, 61, 62 & 63.
Geoff