Hi Kevin,

Yes, it gets muddy quickly, starts in my brain and sometimes leaks out onto the keyboard.
I can see that, where you have quoted me, I have morphed from referring to "Y" bikes in the first half, to referring to "all" of the 1970's in the second half, all in one sentence!

Since I've been "collecting" these unit twin serial numbers, I hadn't come across a 4 digit 1970 "Y" and had assumed that they were all 5 digits, as are the normal 1970 numbers.
You've obviously got two examples with only 4 digits, proof enough for me.