Followup on the IHC Model S Speed Truck tire change: Turns out that between clincher tires and split rims there was a (very) brief period of solid rim, tubed tires with retaining rings. Precursor to both split rims and widow makers. Retaining rings are not a concern -- they come off easily and without danger...but the lack of a split rim makes my master plan null and void.

Thankfully there's a tire shop here that knows exactly what this is and can take it from here.