I have no idea what it is sorry, but I'm sure it's not a Bosomworth.
You're most likely right about the serial number being related to the date it was made, but sadly lots of builders do/did that so it's not so much of a clue.
The lugs and fork crown look more to me like an English frame than Italian - how do you know it's an Italian threaded headset? They are almost identical to, and interchangeable with, English headsets. The bottom bracket threads would be a better indication, as Italian threaded b/bs are unusual on anything made outside Italy.
I recently sold a frame with similar forks (mine undrilled) and lug work. Was rode by a NZ junior sprinter 20 or so years ago. Would take a guess and say its a New Zealand made frame which I'm pretty sure mine was. But check the BB, if that is Italian at least you cancel out quite a few makers.
Thanks for your help. They guy I got it off said the headset was italian threaded. I'll try find out more about the bottom bracket if I can. Would photos of any specific area help at all?
Thanks Musta, I was thinking NZ made too, which is why I assumed maybe a Bosomworth. I'm not sure what to check for on the bottom bracket to see if it is italian. I know the shell width is approx 680mm which I don't think is the standard for italian shell widths.
The headset I have for it is a Tange Levin CDS and the size is 30.2 - 26.4