Tap a flat blade screwdriver into the slot to slightly spread the clamp, I've had to do this to a few, don't over do it tho, then it can be too loose. If they're too loose or hard to get tight, squeeze the clamp a little in a vise so they just slide on the base. If you use a small screwdriver, you can position it so you can spread the clamp and slide the ring on the base with the screwdriver in place, then wiggle it out after you have the clamp screw in place against the front of the base slot. I have over 30 rifles mounted with Sig Zees, they're my favorite Weaver style ring, Midsouth prices the matte under $29 regular price which is better than most sale prices elsewhere online.
Tim