Looks like a mid level Italian frame. Olmo, Frejus, Lygie, Mondial, etc. used that collar on the seat-tube but not with those stay-tops. "Extremely light" does not mean much without an actual weight. Put it on a scale. If the frame alone is under 4lbs, it's extremely light, but I doubt it will be that.
A 60's bike would most likely have a headbage, so if the holes have not been filled it may date it to the 70's.
Is the BB shell Italian thread?
Hopefully you can find a more appropriate fork for it.