Way back, motorbikes were more like bikes, some of these photos make me think those tubes were made in a way they could accommodate a gas tank if they had to. I like the appearances of some of the old bikes and comparing them to the old motorcycles/motorbikes; they seem similar.
Check out how this motorbike has the gas tank, having said this, the below frame I'm sure is specific to a motorbike but nonetheless, one can see the similarities; I guess the top tube may have been aerodynamic:
1912:
