A little more info is required to confirm it's a commerical BSA action. A picture of the left side of the action is need as well as the top of the receiver/barrel. How long is the barrel?

I believe you have a commerical Lee Speed by BSA from the butt socket markings on the right side as well as the 3 crossed rifles on the barrel knox. The "F.C." stamp on the barrel is one I have not seen before. The barrel is stamped "nitro proved" (?) although faint which indicates prewar1.

Lots of modifications....front site change that you mentioned and I see a Winchester elavator sight added...dovetail most likely cut in barrel for this...stock looks to have been replaced with a military one. These parts are tough to find and pricey....I just leave the rifle and enjoy shooting it if the headspace and bore are good.

Ron