Some pictures to assist.
The 'split-rim' Model T wheel uses a demountable rim that needs a tool to remove the tire, by 'splitting' the metal rim. That rim is used on wood spoke wheel that holds 21" dia. tires, normally the tire size is 4.40 x 21, balloon with hard beads, with low pressure 32psi.
The clincher rim demountable is made for 30" tires, where the tire is stretched to go over the rim and the cupped rim locks the soft clincher bead. These tires are normally 3 1/2" x 30" and are high pressure 65psi.