That is the continual problem. The Clearance on the BEIII slide behind the dovetail cut is too short. The Novak-style (1911) cut is .466 with a standard 65degree angle. The stock rear sight has a distance of 0.469 from the dovetail cut to the back, leaving it only a hair before you hit the slide cut.
I spoke to Novak sights and Brownells and BOTH said they do not make or carry a rear sight that fits. They also do not make a front sight that fits (why Magnum Research has proprietary sights without other options is beyond me).
The problem, as I and Bugbottle above have found, is that any adj sight when being lowered will hit the frame before it's zeroed. If using a dremel works, I'll try it. If this fails, I'm taking gun to gunsmith to have the slide cut milled a bit (there's a lot of room back there) and then a bunch of sights will fit, maybe only needing to be filed to .466 for the cut.