It's a cool gun, though as far as Parker's go it's not one of the more valuable Parkers.

When you say the butt is not original, I assume you mean the buttplate? If so, it is an original type so unless it has a non-matching serial number or something I would think it is original.

Assuming the barrels have not been cut and they are in good shape inside it is probably a $700-1000 gun.