mag41vance,
If indeed the charge is 20 grains—IT AIN’T AA#5. Twenty grains of that stuff would be a DOUBLE CHARGE! I suspect that would be enough of that powder to cause problems!!!
Since there are only a few powders that would allow a 20 grain “safe” load with the 210 grain bullet, it could be one of them. They are Winchester 296, H-110, 2400 and Winchester 630. Although greater than a maximum load, someone might have stuffed that much AA#9 in it also. Winchester 630 has not been around for a few years, but if they are older reloads, that is a very good possibility. A Winchester powder would be my guess, note I said guess…
So, in all safe loading practices, I would really not try to identify the “MYSTERY” powder, but start with a good slow burner, as the ones mentioned, and work up your own load as recommended in a good reloading manual or two or three. Compare data and go from there. Good-luck…BCB