I found the problem this evening while taking another look. So I'm going to document here for anyone else who may have a similar problem because I found nothing posted on this particular issue anywhere else. It turns out that, I guess, over time reversing switch lever on the mech. had bent after being tripped back and foward for half a century. Not by much, but just enough to cause a problem with the A1 selection. Following the "REVERSING SWITCH 1" adjustment in the service manual (page 2464) fixed it.