That's a sweet looking bike with a cracking paint job. I hope you enjoy riding it!
Don't know if you're still around, but the reason you got no replies is probably due to the bike being very rare and it might be that nobody has encountered another one of these.
It is interesting that you've picked it up in a Dutch marketplace. From Zullo history:
"
In 1985 he crossed paths with top Dutch professional team TVM. This would lead to Zullo being the frame supplier of the team from 1986 until 1992. The team was well known for such top professionals as Phil Anderson, Dimitri Konychev, Jesper Skibby and Scott Sunderland.
The TVM team utilised the best of frame technology available at the time. Using everything from Columbus SL, SLX, MAX and MS on road, time trial, track and even cross bikes. Zullo frames would be ridden on the roads of Milan San Remo, the Northern Classics and the Tours De France, Giro d’Italia, La Vuelta and World Championships.
In 1994, Zullo would move into the production of tig-welded aluminium frames. Eventually this would lead to the production of combined aluminium/carbon frames. In 2003, Zullo was able to offer made-to-measure full carbon frames. Nowadays, Tiziano Zullo is still involved in the production of some, mainly steel frames. Alongside this, he is still responsible for all the frame graphics and some of the frame painting."
Another peculiar thing is that the bike is made out of old-school lugged non-OD (?) tubing.
If that's the case you've got 0.8mm straight gauge cromoly throughout, apart from fork legs that are 1mm.
What's odd is that if the bike was built in the very first years of the 90s, the builder would have had access to various OS tubesets, so it's interesting that he opted to use a non-OD lugged one. Could be an older frameset?
Or it could be that he opted to use the non-OD tubeset because it was half a kilo lighter than Oria's top or the range OS MTB set, albeit, surely the lugs would add some weight compared to a fillet brazed frame.
It could have been a one-off build in late 80s-early 90s for a team racer at the times when offroad racing was gaining momentum? Or a prototype build that was tried out and not used for pro racing (going by the pristine paint condition)?