Hello:
I have purchased on ebay a 3x4 23 graflex roll holder for my Crown Graphic with a graflok back, but it does not fit, it is slightly wider. Any idea why is that?
Thank you

You may have accquired a roll holder for the Graflex back as opposed to the more common Graphic,or vise versa.
The part numbers for Graflex type backs, with a groove light trap and slightly larger, are #1150 for the 6x6, and #1147 for the 6x9 format.
For the Graflok back on the Speed and Crown 34 series cameras one would want to accquire the roll holders part numbers #1247 for the 6x9 format and #1250 for 6x6. These have a raised ridge light trap that drops into the slot on the camera back if if is a graphic type.
Though not as common, some Speeds and Crowns came with the Graflex back standard to the SLR series cameras. The Graflex back That fits Crown and Speeds has slide locks similar to the Graflok back but lacks the quick release locking spring arms. One must have the proper film holders for either type as they are not interchangeable as you seem to have discovered.

Dear troblemaker
Thank you for your kind reply. Unfortunately I cannot find a part number on the graflex back, but your explanation seems to identify the problem.