Thanks once again for your input everyone! This forum is terrific!
To answer a few more points that have been made:
1) No adjustment provision on the vacuum advance unit.
2) Put a hand vacuum pump on the advance and it moves the plate in the clockwise (correct) direction. I compared the rate and amount of movement to the 440 distributor that had been in the car previously and was known to be good. They look to act the same.
3) The loss of advance is as follows: Disconnect the vacuum advance hose with engine idling and plug the end of the hose. Set timing to 8-10 degrees before TDC with a light, with engine at idle (about 800 RPM according to the old time RAC tachometer in the car). Reconnect vacuum advance hose and slowly increase engine speed. Timing retards to well after TDC. If I disconnect the vacuum hose while this is going on, the timing recovers to about 12 degrees before TDC, a point consistent with what the I think the mechanical advance should be doing by then. I hope I've stated this better than I have before.
Anyway it certainly appears to be an electrical issue, not a mechanical one. Tomorrow I'm going to switch the two lead wires just to see if it makes any difference. Ironically, one point I'd not made before, as I didn't want to be beating up on an eBay seller, is that I'd emailed him to question the wire colors on the new distributor comparing to my old one. (Orange and black) They seemed the opposite of what I found on my old 440 distributor. The response I got was that it was wired right and to just plug it in. This may well be a non issue and I'm just overthinking the whole deal.
Next step will be to install the distributor Don Frelier was kind enough to sell me cheap. Thanks again Don!