Howdy, Cuzzin!
As Del wrote, the elastic can be early. There are some photos of early military ones around: they're some type of cotton or linen, and had (IF at all) a type of adjustment buckle that had 2 prongs which would pierce the strap material - kinda like some of the vests or pants buckles sizing straps, only bigger than the vest/trowser types. I've also seen straight straps that use a single pants button. Not "X" OR "Y" type.
As to which was more popular, well, probably what type the sutler(s) carried!
Personally, the "Y" type with the back "leg" made of elastic is, to me, both the most comfortable and most secure.