Looking at the back it comes with a 2x3 roll film folder but the back opening looks to be a 3x4 or 4x5. Is this a home made conversion to allow a 2x3 roll film holder to be used? What film holders would fit this setup?
Well, the roll holder says that it is a 3 1/4 x 4 1/4. If you don't believe me, look at it. It is not home made, it came from the factory. It has a slot, not a ridge, at the insertion end so it will fit a 3x4 Graflex back. I'm pretty sure, could be mistaken, that your camera is a 4x5er.
It won't fit your 4x5 RB Ser. B. It appears to have a Graflex back. To verify, check whether the bits of metal below the gate have a slider (if so, Graflok) or a trough (if so, Graflex). If a Graflex back, it will have a ridge at the insertion end that mates with a slot in the film holder. I can't discern the ridge in your photo. If Graflok, it will have a slot that mates with a ridge on the film holder; should be visible it there, I can't see it.
Early modern (knob wind film carriage, "120 Roll Holder" printed on the back of the shell) Graflex brand roll holders were hybrids that fit Graflex and Graflok backs. I think, could be mistaken, that they were made to fit 2x3, 3x4 and 4x5 backs. Very scarce. I have two 2x3ers, don't recall seeing 3x4 or 4x5ers. Later modern roll holders from Graflex fit only Graflok backs.
If your RB has a Graflex back and you must shoot roll film with it, your choices are: replace with an RB whose Graflex back was replaced by a Graflok; have someone, e.g., SKGrimes, replace your RB's Graflex back with a Graflok; get a 3x4 Graflex reflex with a Graflex back so that you can use the roll holder to shoot 2x3. Graflex made roll holders to fit 4x5 Graflok backs. Machining one to fit a Graflex back might be possible. These beasties aren't that rare, but if you want to go that way check with SKGrimes whether machining it to fit a Graflex back is possible before you shoot yourself in the foot again.