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I recall seeing a post on here about a few rare Vit's with different head tubes although I may be getting confused with one of the orange alu models. Time for some searching.
<Edit> Here we go. Although that one has a seat clamp.
Just to muddy the waters - My Vit T also has an 1 1/8 head tube and it has no welded seat clamp ... because it sheared off on a ride in the lake district in about 1995 and I had to finish the rest of the ride standing up.