Differences between 20" RCA A51A CRT's?

[damen]

It's been a long time, but I think the last digits has to do with yoke differences if these are bonded yoke types. May be impedance, or if the wires plug onto the yoke, or the yoke has wires that plug onto the chassis. May also be a difference if the corner mounting flanges are flush with the front of the screen or recessed slightly.