I purchased a 20" floor tom with the rest of my old kit back in the '70's. I sold it, and recently bought it back. I got it from the factory originally and it was set up as a floor tom, with four legs.
So I got a couple of 20" tom-tom heads and found they didn't fit. The drum is apparently set up to use a 20" kick drum head...the collar being different.
I never changed a head back when I originally had it, so it was somewhat of a surprise.