Just the way the machine stamped them out. In the earlier, days the stamping machine added the bulge so it could accommodate both standard and parallel strainers...? Would make more sense to manufacture something as simply as possible, from a production point of view, but a flat hoop sits flat, so that is why they went to a flat base later on. As for the shape of the snare guide, machines wear out and you replace them.

This is all B/S and conjecture, but it makes more sense than "The drum Gods said it must be...".

Both hoops are correct for their respective years. Ludwig first manufactured only the hoop with the drop gate to accommodate all their different strainer designs, ie: P-83, Supersensitive and the Classic strainer. Both the SS and Classic strainers required more gate opening for the way they lowered the snare wires. If memory serves, the "drop gate" hoop was used on the legendary Jazz Festival up to around 1964.

I've not checked a '58-'59 Super classic and '65 Supura bottom hoop. Is there any difference? Later, I'll check them.

Mine:

'59 Transitional Super Classic NOB "drop gate"

'65 Supra / COB "drop gate"

'67 Super Sensitive / COS 1.6mm "round"

'68 Acrolite / COS 1.6mm "round"