Of course the 1969 "normal" numbers went at least as high as 22610 so it's another mystery as to why these 7000 numbers were still around at the very end of the season.
The 1969 'normal' bike numbering started at 10,000 and the 1970 bikes all had a zero or one followed by 4 digits.(ie the numbering started at 01000 or similar). Neither at 101 like the dating lists say.
Neither scheme has provision for a four digit number.
All those Y bike numbers must have been chosen from the 1967 despatch books.
I am not convinced they were picked in any particular order also. I think there is every chance that 7879 number was from about March of 1970.
I really have no idea. The numbers just seem all over the show.
We would need to see the 1967 and 1970 books to have a fighting chance of working that stuff out.