I haven't been able to find out the grip screw threads for the wood grip Mk.1 from my Webley contact, but he did send me the following:

The slant back thread was 0.150" dia x 36 T.P.I. which was the same as on the revolvers.
The 1939 junior was 4 BA (0.142" dia x 38 T.P.I.) A 4 BA mild steel screw would screw into the 0.150"x 36 T.P.I. thread over such a short distance.