Measure the width of the gap between the rails. If it's 3/8" (Anschutz), you want the HB6 stud adapter. If it's 5/16" (Freeland), you want the HB6A adapter.
Depending on your preferred mounting method, you may also want to consider the BT28 AFAR Rail Kit, which will fit either rail.
I've used both the HB6 and the AFAR, and they both work as intended.