Got it.
UnbearablePanda was right. Maybe that's the difference between the 500 and the 500a. It came right out, no problem after I completely removed the stock. As I gradually loosened it, the housing got looser & looser but wouldn't come out until removed. There were already 2 washers under the bolt head. I tried adding a 3rd & put it all back together but nothing changed. Then I put it back together without the trigger assem. & looked at the threaded part where the bolt threads in. It was shy of even coming flush by a few threads. So I removed the 3rd washer & reassem it again to retain full thread contact into the receiver. I think the situation is that the rear,rounded tang on the trig/assem fits so tight in the stock, that there's no swing room for it to lift out. The front edge of the trig/assem hooks into the receiver so the rear must swing in an arc as it's lifted. The only other way to change things would be to file away a clearance angle on the tang which might weaken the perfect fit it now has. I want to thank everyone for all the tips. After today I feel like I could take it apart & back together blindfolded.