The base that I found in my Springfield box is marked "SP" but it is nowhere near a match for this receiver. It has a flat rear portion with a lateral groove machined through it. The Buehler base has a radiused rear portion that matches pretty closely. I am going to see if I can machine a new hole between the front screws to move the front ring rearward...
The base i have may be for an A3 and I may not have an A3- or the other way around?
