Referring to the '67 production books with the hand entries on the inside of the front and rear covers with dispatch dates in January, February and May 1970, I can now verify the following information regarding those "Y" production numbers.
The numbers listed in the inside covers also appear in the body of the book. The numbers in the normal pages of the books had dispatch dates already amended one or more times so there was no room for further entries. Those specific production numbers in the normal pages, which are also listed on the inside front and back covers, are not blank lines.
As an example: LA 12212 had despatch dates in 1967 and 1968 on the normal pages in the book, then again in 1970 on the inside front cover.
As a result, it suggests that there are NOT two machines out there with the same number (ie. one with '67 features and one with '70 features).
1967 BSA Wasp 1967 BSA Hornet (West Coast Model) 1967 BSA Hornet (East Coast Model) 1968 BSA Firebird Scrambler 1968 BSA Spitfire Mark IV