(all the 1970 Y bikes have five digit numbers),
I don't think that is the case.
All the normal 1970 serial numbers have the 5 digits for the 1970 season. This was achieved by placing a zero at the start of the number if required.
I have a couple of 4 digit Y bikes listed here.
I will see if I can dig up the numbers.
7879 is one, 7031 another. (Edited.. added 7867)
I am guessing it was because they were either picking up numbers, - or spare numbers.. from the 1967 despatch books.
The 1900 number is really unusual as you suggest though John.
It is very low if they were using the 1967 books as a guide.
I thought you may have been tempted to add the engine to your collection.