Vibration and leverage are generally what I rely on. You obviously understand the importance of using a proper fitting screwdriver (there's nothing worse than mangling a screwhead), and you tried penetrating oil, but I would add mechanical vibration to the mix which often is what's needed to overcome a frozen thread. Gentle tapping on the handle end of the screwdriver, along with alternating between tightening (yes, that might seem counterproductive) and loosening the screw, anything to establish some movement. Adding leverage by using longer screwdrivers also helps, but with such small screws you have to be careful as it is easy to overpower them and break them. At that point you'll need to drill them out, which isn't that difficult, and possibly re-tap the screw hole.

I had to look up Plusgas as it wasn't a brand that I'd heard of. Given it's supposed to be a quick acting release agent, what do the directions on the can say? I would think that a repeated cycle of application and soaking should do the trick, and then repeat the vibration technique. If you want to keep the original hardware then patience will be the key. Heat is also an effective technique, but not on a typewriter; the amount of heat that you would have to apply to that small area would probably end up damaging the paint.

All I can add is that tapping combined with gentle turning both ways and penetrating oil/solvent will free the thing eventually.  Just been working on the corroded chuck of an old hand brace - it took over a week to free it.  Plenty of oil, tap tap tap, turn back and forth tap tap, more oil, repeat, tap tap, turn turn, repeat repeat....

Fleetwing wrote:

I am trying to remove the four screws on the back of an SM-7 to get at the tab brake -- I am pretty sure they were painted over at the factory when the machine was assembled.

Those screws are not recessed, but they are prone to rusting if the machine was stored in a damp location for any length of time, so I'm willing to bet that it's rust that is giving you a hard time and not paint. I've never had issues with SM7 covers when the machine was in good condition, but have had to use quite a bit of leverage to remove ones that slight rusting.

I've ordered a screwdriver with a 4mm wide head 0.5mm thick. Patience is the key I think. It will come out in the end!