Gary, my apologies for any elevation in your blood pressure that I may have caused, that was not my intention at all.
Since the mystery of the BSA unit twin serial numbers caught my attention, I've tried to sort out the truth from the quagmire of misinformation surrounding these bikes. My notes consist of quotes from a very few select sources whom I consider trustworthy regarding this subject, your quotes likely outnumber the sum of all the others. We can't thank you enough for sharing the information you have gathered from the factory records.
While I'm sure that what you have posted is as accurate as you could determine from the records, there are a few places where I am having a hard time connecting the dots.
Although I didn't spell it out in my initial post, this is the main one, these two quotes seem at odds with each other. What is the missing part that makes them both accurate?
“The '68 production book, and I am verifying '68, not '67, has dispatch dates of these '67/'68 hybrids (for lack of a better term) as presented previously. They were dispatched intermixed along with the true '68 SB Spitfires"
"The entries are in the 3rd '67 book (#276), in the middle of the book, after several blank pages. They are not in the '68 books.”
You've already covered the other area that I was unsure about. Hopefully, at some point, someone will be able to confirm whether or not the SA "Y" numbers had already been used during 1967 production as SA "-Y" numbers. My thoughts are that while the last 1967 model dispatched during the 1967 season was SA 16614 (5/16/67), I have to assume that they continued to build 1967 engines/bikes for a few more months based on the first quote below. These would have been "-Y" engines. Now, did they deliberately not use all the numbers in that range so that they would have numbers left over to use on the SA "Y" bikes, or did they just miss 478 numbers in error? Neither of those possibilities makes much sense. Those 478 1967 numbers must have been used, and then came available for re-use, likely for the same reason as the LA/RA/TA numbers re-used in 1969/1970. A catastrophe, with perhaps some resemblance to what's been suggested by the BSAOC and others, would be the ticket.
Note SA 11577 Y below, was that the first of the SA"Y" bikes in book 276?
"Book 276 starts at #16656 and goes to #18601 with it dispatched April 24, 1968 (yes '68, and they are not noted as "Y" bikes.”
“Hybrid '67 SA's start at #11577 (dispatched 4/18/68) and randomly go to #17892 (dispatched 4/18/68)."
Again, thank you for everything you've done to firm up our knowledge of these bikes.