Dan, I'm all for solidarity among Dans and I certainly don't want to cost you a sale, but the 2x3 RB has a Graflex back, won't accept roll holder made to fit only a Graflok back. That's your Singer Graflex roll holder, which is probably badged RH-8.
OP, look on eBay. Very early "Graphic 23" roll holders are hybrids and will attach properly to a 2x3 RB with a Graflex back. I have one for my 2x3 RB.
If you look at your RB's back, you'll see a metal lip at the right (insertion) end of the gate. This mates with a slot in the roll holder. Roll holders for Graflok backs have a ridge at the right end that mates with a slot in the back. Hybrid roll holders like mine have a slot and a ridge.
eBay sellers often don't know what they have. I found my hybrid by looking at listings for 2x3 Graflex roll holders. I found a later roll holder for Graflex backs only (slot, no ridge, keep an eye out for one) the same way.
Good luck, have fun,
Dan