Finally we found out that this Norton lives in Italy and is owned by Giulio who now sent me a photo of the left hand side of the machine; clearly a very nice Norton! The colors of the bike and the wall are amazing.
Giulio made a modification that I have been thinking about for a while; the inlet valve guide may wear at an alarming rate and diverting the crankcase breather to the valve guide should get enough oil mist up there for proper lubrication.
Note the early type of cylinder head; the photo below compares an early head (on the right) to a later one.
...and a comment from Martin:
"This is a nice picture of an attractive early Model 18 although quite a lot of it is not 1924, as I suppose happens with such bikes. But dating the head is an easy task as Norton conveniently stamped the date of manufacture in small numbers beside the inlet stub, on the thick part where the inlet rocker support is bolted. This is the year and month of manufacture. I have an early head stamped 2310 which is October 1923... This is often not very visible as the numbers are small and often obscured by the black coating on the head, but a polish with a Dremel or similar will reveal it."