Hi, cheers for the welcome..
The haynes manual I have is covering 1967 thru 1981 C50, 70, 90. There are exceptions for my variant (k3) listed as usual in the rear.
It doesn’t explain the tank assembly though, or why it’s so different to every other one we or anyone I’ve spoken with knows!
Am off on a mission today to cruise the breakers yards. Hopefuly this will yield something worthwhile. If not it’ll be a good day out smelling grease and oil.