Butt pad screw size?
 

Does anyone know the original screw size needed to attach the butt pad? Are they common that any standard hardware store should carry? Thanks.

Original pads on guns of the era of your gun are so scarce that it would be better to try different screws until you find one that goes in but fits tight. The base of the screw head should be flat. My favorite Griffin and Howe pads have a steel washer so the screw does not bottom out on hard rubber.

Thats a very good point Bill,I also put a washer under the screw of all my buttpads,espeacially the silvers type with the hard rubber base(like your G&H) that stuff loves to crack....for screws where they won't be seen ,it's #12-14 ,1 inch Robinson pan heads

Is that the same as a pan head sheet metal screw?

Pretty much it.