Thanks Norm. I won't keep the mystery for too long now, as I will need to start on the frame etc soon. I'm sure some visitors/Members will know what it is.
Cyril Grange's book was a significant reference for many years. first published in 1929 and the ninth edition in 1954 (mine's the latter). I have the 2nd edition (I think) as an 'E' book which revealed the clutch info.
Brief update-
Working through the power unit and associated parts, I started checking out the Fuel Tank. An irreplaceable item for this machine, so I was pleased to find it bone dry inside. A thin layer of dried up petrified fuel/oil was able to be scratched away from the inner surface. I went to my 'Scope on a Rope' to check it out. Images are not great quality as copies, but gave me a good view-
So I started to check the outside soldered joints and the level of rust penetration to ensure no perforations-
Nothing that can't be put right cosmetically and with only 2 small/shallow dents in one end and the dents/marks where it is fixed to the frame.
Only thing to repair is the outlet boss, where the Tap/filter must have been hit at some time and bent/cracked the old lead soldered joint-
Tinned Steel plate is not very thick, but certainly the plating helped keep the bad rust at bay and happy with the initial external clean up-
So started on inside using pea sized shingle dry to start with and does it well -
Will do several more 'shakes' with this, then when I've sealed and repaired the external bits, I'll change to fluids with the shingle and a final washout.
The tank is 1/2 imperial gallon (2.27 ltrs). Fuel pipe is 1/4" copper pipe which I will need to replace (to save repairs and cleaning)......tbc