The Service Notes posted by Andy are dated 10/11/66.
These Service notes were referring to bikes made before that date (in 1966) but were 1967 models. The Lubrication and ignition updates to be performed were for these 1966 made bikes. Those bikes with a "Y" or "X" suffix, also manufactured in 1966, already had the improvements installed plus the suffix, to avoid any confusion as to which bikes were to be updated.
"Speaking to Alistair Cave (who was head of the BSA production), the machines were sent to the USA in 1966, but returned unsold."
I think that Alistair Cave in referring to "machines returned unsold", he meant that they were returned to Nutley in New Jersey, unsold, for the modifications to be performed. Not returned to England to be updated at huge cost to the importer and/or the BSA Company. That is why the service notes were sent out to the importers for the updates to be performed "in the field".
My own recently acquired Lightning has matching Frame and Engine numbers - A65 LA 7876 Y
This is a 1967 Number with a "Y" suffix.
This bike has a compliance sticker
on the steering head of the frame showing it was manufactured by BSA Motorcycles in 02/70. The engine number is included on the compliance sticker
This bike is clearly a 1970 manufactured motorcycle as it has all the 1970 features. The frame has the 1970 swing arm and the horizontal tubes for attaching a fairing welded to the front of the steering head. The engine has all 1970 features, 12 point cylinder base nuts, 3/8" base studs. thicker base flange, piston oil pressure relief valve. The clutch cable enters on the top of outer cover. The front brake is the 1969-70 version of the twin leading shoe brake. The clutch and brake levers have the holes for attaching rear view mirrors.
It is not credible that this bike was originally manufactured in 1966-67, exported to the USA, returned unsold at great cost to the BSA Company, then the engine, frame and front wheel were all removed and replaced with updated items with a new compliance plate displaying it was manufactured in 1970. Then it was again exported to the USA to be sold!
What happened to all the engines, frames and front wheel assemblies?
A chap in the UK re-imported a "Y" series Lightning with Engine number A65 LA 7726 Y with a compliance sticker
the same as on my bike with a manufacturing date of 02/70. This is 150 numbers before my bike. It seems likely that at least a block of 150 "Y" series bikes were manufactured in February 1970. There is a restoration build started by "England-Kev" on:https://advrider.com/f/threads/the-bsa-a65-a50-series.1171845/
of his "Y" series Lightning.
I am sticking with my theory at this stage.