Interesting issue, I've a frame around late 1980s as per usual when I find an old frame / bike / bits its been hand painted so all original decals & marking are lost, I've one frame which has two parallel fixing holes i.e. east west not north south, so I'm slowly building up a case when ever I view photos & see the same fixing I'm jotting it down as a possible contender, as I'm hoping with that list (at the moment is rather small) & the seat post diameter I can narrow in down further.