This is definitely a 1984 variety of the GAP frame. I have a vintage Tretubi Basso I used for training 30 years ago. I also had a GAP frame Basso which I sold.

The serial number denotes the year it was built...first number indicates the year it was built (4=1984) as does mine. The derailleur cable stays under the BB are the same as my 1984 non-GAP frame. And the geometry is correct.

The dead give-away for the GAP frame is the chromed out stays and forks. The Tretubi version, even though it had the same geometry, did not have the chromed butt ends, chain/seat stays or forks. Basso varied wildy during this era, as one poster mentioned, but chrome on the chain-stay or fork butts,

is a dead giveaway.