I bought a second-hand Marlin .30-30 and it had high see-through mounts and a cheap scope on it. Decided I just wanted open sights for now. Went to remove the scope, rings first, and can't find a hex key that fits the bolts. I went through every close size in my American and metric key sets, and used some other loose ones I had. All slightly too small or too large. Is there a standard size I should look for? Don't see a name on the mounts, scope is a Simmons. Thanks