I've wondered what this hole is for sometime, doe's anyone know?

The Enots necks on Nortons have them too. I'd always assumed that it was for a locating dowel at some stage in the production process.

,... and there was me thinking it had been drilled to reduce weight.

email (option): cruiserchooser@hotmail.co.uk

There is olso an unidentifi hole in the upper chain guard by the stay Gr Theo

email (option): oldbikes@kpnmail.nl

According to Ian that hole was used in combination of a shorter version of the upright mudguard stay. This was done to reduce the thickness of all parts fitted to the stud when the bracket for the field-stand clip was added there. He says he has a photo of it be I never saw it.

Henk

email (option): ahum@quicknet.nl

Just thinking outside the box.
Could the hole be used to attach a tag on a string that has the information about the M20. The tag would be placed there when the motorcycle went in for repairs. It is in full view of who ever walked up to the motorcycle telling them that maybe, "DO NOT START NO OIL", need to have motor checked, or NO BRAKES....
Just thinking while drinking some coffee this morning.
One of the few, Frank USMC RET

It could be for centering the plate in the press so the bends were in the right place, minimizing the amount of steel used to make it. When the sheet was pressed into strips the hole could be pressed at the same time with the folds being completed in a hand press with little or no wastage.