So, exactly what do you consider HIGH rpm.

Is it possible for you to consider that at 50 mph in high gear you are no where near high rpm.

If the previous owner installed a larger transmission sprocket, as it is common to do these days, the rpm at 50 mph would be even lower.

Can you understand the concept that you are measuring dynamic cylinder pressure when you use a compression gage. Can you understand why the gage measures a lower pressure when the throttle is closed? Can you make the step in understanding what you are doing when you open the throttle from the 1/3rd, or less, throttle opening normally required to maintain 50 mph in high gear and open the throttle to 3/4. Do you understand shifting to a lower gear acts as a lever, or torque multiplier, and reduces the strain on the engine, just like your car does when it automatically shifts down a gear in the same conditions you describe.

Add to this some other problem, even if ever so slight, and you have the perfect conditions for detonation, especially if the fuel doesn't have a high enough octane rating to prevent it.

Remove your plugs, and with a small focused beam, look at the cylinder bore. Do you see the pock marks on the top of the piston consistent with detonation? Do you see any scuff marks on the cylinder bore?

And yes use the clutch as a diagnostic tool. When it seizes pull in the clutch. Is the wheel still locked? If not your problem is in the engine. If it is look at the clutch and transmission.

Let's repeat this:

These BSA's are NOT hardleys, they should NOT be allowed to let their engines run hard at low RPM's, lugging the engine.

At 50 mph in 4th gear, you are lugging the engine. BSA twins don't like low RPM.
If you are running the engine only within that 2-3k window, you are lugging the engine.
But I'll admit the wrong tach may have been installed, but that would point more toward the bike not being quite as original as you may have been lead to believe...

This said, I had another A65 decades ago, fresh bore job, and the engine got hot at times and heat seized.

Sure, check the timing, absolutely, but you may end up taking the cylinder off to have the clearance checked...
You may want to look closely at the ring end gap also. I've heard of rings getting too hot, expanding too much and seizing the "piston" in the bores also.

If the engine is vibrating your hands, "unsatisfactorily", and you are lowering the RPM to get the vibration away from your hands, you are hurting your engine.

Go find that guy with another A65 who wants to go for a ride, and the two of you check each others ride out, see if'n the two of you can figure the problem out?
Or find the local BSA meet, and see if'n the fellers can sort your bike out?

HTH....

I read and hear that there are modifications concerning the side to side wandering of the crankshaft and balancing the crank that need to be addressed to make these motors reliable and better runners.

The thing that I seem to be failing at describing is the fact that I am not lugging the engine. The throttle is 80+% open when the engine seizes. I do not have the tranny in the last gear and only 10% on throttle as your post implies.

I think a good rule of thumb should be more like don't apply more than half throttle BELOW 3000 rpm. The sweet spot is between 4000 and 5000. You should feel it really come alive at 4500. If you want to be conservative, shift before 6000.

Seriously.

I'll bet that even after all the help in previous posts, that you cruised at 45 mph in "last gear" Ah well. Dick