I'm having trouble figuring out which 120 roll film back will fit my 2x3 RB Series B. The back has a single sliding locking bar; the other bar is fixed. Along the other two edges, the back has metal edges. As a result, there is recess about 1/8-inch deep measuring about 78 mm x 110 mm. The camera came with what appears to be film pack holder that is the right size to seat into the felt-lined opening. Sliding the locking bar firmly seats it. But I want to use 120 roll film holder, preferably in the 6x9 size to get the largest negative possible. I would appreciate all advice. Thanks.

your looking for a slotted film holder. it has a recessed slot instead of the raised ribs. in 2x3, that a rare duck !
i looed through current and past ebay auctions and found nothing, even did a quick google search for you and turned up nothing


sorry

Your GraflexSLR has a Graflex back. Graflex roll film holders for Graflex Backs are rare and pricy.
See: https://graflex.org/helpboard/viewtopic.php?t=6356