I would tend to agree with Jonesy on that one - M5S1 with a M5S3 motor change. I do like the blocked off carby cap though - I wonder if it was a factory job?

You mentioned sales receipts for a Model 4, Deejay? Can't help there, but these were generously supplied to me:

SN: 225,146 - 6/1/1959
[Linked Image]

SN: AA18,716 - 6/10/59
[Linked Image]