I recently aquired an older Lakefield 64B from my Dad, likely made in the mid 80's, it was the first gun I ever shot. I'm trying to find scope rings to put a better scope on it, but I've gone through 2 different sets of ⅜ dovetail rings and they are too loose. When I took off the scope and mount that was on it (an old Bushnell 4X custom .22) a piece of wire that my Dad had used as a shim fell out. Are the grooves an odd size? I tighten the rings as much as I can and they still slide back and forth. Everything I've been able to find says ⅜ rings should fit. Anyone else have any advice?