The adapter plates provided an interface between the gearbox and the propeller hub, support a seal and thrust bearing for the bevel gear, and provided for mounting the large bullet shaped canister. In the Curtiss manual, the plate, bearing, seal, and gear are called "the power gear assembly."

There  are two different adapter plates, one is round, the other is semi-triangular. The latter type seems to be more prevalent.

Here are some photos of the "Power Gear Assembly".

Complete Assembly

Assembly with "Power Gear" (bevel gear) removed. The thrust bearing and retaining ring, and seal below are visible.

Completely Disassembled Unit

Adapter plate on left, seal at top, thrust bearing on center, retaining ring at bottom. Gear on right.

Most homebrew prop pitch rotator builders have thrown most of these parts away and just used the gear. This is not a very good idea. Using the entire assembly is a good idea as it can seal the top end of the rotator and provide a bearing support for the gear which is what is connected to the mast. The bearing is an MRC0017 which is no longer in production, but similar bearings today can carry ~ 2000 Lb of thrust and can easily handle the lateral loads at the bottom of a mast.