Did the serial number stickers on II Plus and Bell & Howell cases start at 0001 like the original Apple II? I just picked up B&H A2S3-001488 that has a rev.3 (removable 16K blocks) motherboard marked 7921 in the upper left corner suggesting early June 1979 assembly. Did the two models hit the market simultaneously? What's the lowest serial number of either model you've seen, and do you know the motherboard date?
And on a somewhat related note I just saw some online photos of a 1979 original Apple II also with a motherboard marked 7921 in a case A2S1-28377. The owner calls it a rev.1 but I believe both his and my B&H are identical rev.3 motherboards except for ROMs. I imagine stuffed and completed motherboards didn't sit around very long at Apple before being put into cases, so it's interesting to match those boards up time wise and appreciate how many IIs had been made by the time the II Plus (and B&H) rolled out.
I have Apple ][ Plus with serial A2S2-003207 and mb dated 7930. I have seen little lower serial (something like A2S2-0031XX again with mb dated 7930)
For information about revisions, dates and so on you may check:
Apple II plus production started with rev 3 motherboard in mid summer of '79. Rev 3 boards were already in use on regular IIs since perhaps late '78 and continued on for quite a while on both II and IIplus machines. Serial numbers were changed to A2S2XXXX on the A2plus from A2S1XXXX on the regular II and counting restarted. Motherboards were frequently swapped out for repairs or other reasons. Another way to date an Apple II/IIplus is to take the case apart and look at the keyboard, which usually has both a serial number and a date stamped on it. Serial numbers for keyboards were not restarted and II pluses seemed to arrive after about 29,000 IIs were shipped. I'm guessing that another 40,000 IIs were shipped at the same time the II plus was in production. Someone else may have better information than I do on overall II production numbers. During this concurrent production, the only difference between the machines was the badge, model number, the serial number and the PROMs.