The Mellotron Group

I am trying to work out when #1565 was built. On the old Steetly site it says 2000 M400 mellotrons were built from 1970-77. Models built after 73/4 were 400 SM and had the SMS2 motor control system.

#1565 is a 400S and has a CM10.

Was there ever a  plain 400?
Where does the S fit in?

I would have thought that #1565 was towards the end of production run. Did they produce a lot fewer of the  400SM, say 3-400 and would that then make #1565 one of the last mellotrons with a CM 10 and a likely manufacture date of 1973?

Anybody know?

Mark