I have to agree about the books. I think you have done extremely well given the limited access you had.
This would have died a natural death years ago if you had not been so encouraging and forthcoming with the information.

You may be right about the uncompleted 1967 bikes but surely they must have been numbers only.

I concede they may have even been a frame that could have been reworked. We did have an issue with some of the early Y bikes being fitted with what appeared to be older swingarms and hardware.

There can be no way a 1967 engine can morph into a 1969 or 1970 engine. Please let us be quite clear on that. It is impossible short of melting it down and starting again.

I admit I am still uncertain about duplicated numbers. That would be a massive risk to take and we have not yet seen a case of that.

How would it be though to have two bikes side by side with the same numbers. (Albeit different style).

Against this we still have the conundrum of multiple shipping dates.

For the life of me I just cannot accept the 1000 bike sitting at the factory story.

I am not even sure we know of a 1967 Dash Y bike being sold in 1970.
Morgan's Spitfire seems to have taken a year from export to retail sale. That seems to be a long time to me. I can see if it did not sell by the end of summer then it may have to be sold the following season at a discount. That would have happened to many.
But to sit in a showroom for two or three years has to be inconceivable.

The Y bikes were made in 1970 and sold in 1970. So that means there is no benefit to BSA in numbering them in 1967 style numbers.
They cost the same to produce, ship and sell etc.

That would be unless there was a law change that permitted bikes made earlier (1967 specifically) to be exempt, or subject to less, taxes.

The savings would have to be substantial though to contemplate such an exercise.

Another possibility is that something happened in 1967 that needed to be rectified.
Bikes showing in the export books that did not exist. Phantom bikes. Bikes existing in number only.
If questions were asked and these bikes could not be made to reappear then perhaps they had to be produced from somewhere.

The later reincarnations were not made in a batch. They appear to have trickled off the production lines. The production may have started during the 1969 season and extended well into the 1970 run.
The very last of the 1970 bikes had the shortened rear brake torque stay. I have not seen a 1970 Y bike like that yet.

Gary also mentioned shipping dates of January, February and May of 1970. That would tally ok.

If these are the two possibilities that we have to explain this situation then I am backing Option B.
BSA exported phantom bikes in 1967... If that is the case then the shipping dates for the first export are bogus.

Is that why BSA had such a good year in 1967. - They exported over 1000 bikes without actually making them ?

It was one of the biggest years, I am not so sure how it compared to 1966 or 1968.

I would be thinking 1968 was a lean one... we know very few true MKIV Spitfires were made as a fair percentage were hybrid MKIV's with 1967 style engine stampings. We also have the story about the 200 or so Firebird Scramblers (debunked I know).

BSA took it upon themselves to start the 1969 Twin serial numbers at 10,000 (or thereabouts).

Similarly the factory started the 1970 numbers with the digit zero preceding the numbers.

If there was a cover-up in progress were these departures from previous years part of it ?

Sorry, - yet more questions.