Folks, if that sucker is AcrasGlassed on there, it is going to take heat -- period. WD40 and so forth won't affect it at all. And it will be more than just the screws -- the entire underside of the mount will be glued in place.
Actually, this is a good idea when you're dealing with a brass receiver that screws might pull out of -- sort of reinforces everything, turns it all in to a single unit. But it is a bugger to remove. By all means, go slow and easy with the heat. Using a tight-fitting screwdriver, try the screws all along the way until they begin to turn. Once they are out, if the base is still stuck in place give it heat, and at each stage of the heating give it a light but sharp rap with a hammer handle or wooden mallet. Eventually it will reach the temp where the AcraGals will release. Know this -- very likely a thin layer of AcrasGlass will still adhere to the top of the receiver. Even with heat, you will likely have to use a very soft scraper -- I use flattened copper tubing -- to scrape it off. If heat scares you after getting the screws out, try putting the whole thing in a freezer for a few hours, take it out, and give the mount a rap or three. Sometimes the uneven shrinkage of the metal surfaces will break the bond. But heat is about the only way the screws are going to give.
I have a good deal of experience at dealing with this situation. Gluing the base on place can be a very good technique -- IF you never want to take it off again. Otherwise, it is a major, time-consuming, but not impossible, job.