I didn't see the ABU prefix in the SCSW, 4th edition, but the closest ones (ABT,ABV) were in 1983-84. If you call S&W, they might be more specific. Could it be ABV?
Welcome to the FORUM from Toledo. My 586 no dash is ACH25xx, from 1983. It is a 4" though. Have a 6" 686-3 also. Gotta love a L frame! Sorry, not the best pictures. Bob