Sheet holders, roll holders and grafmatics made for the Graflex back are wider than those made for the Graflok back so they will only fit the Graflex back. Sheet holders, roll holders and grafmatics made for the Graflok back have a raised light trap that is incompatible with the Graflex back and will cause light leaks. For the sheet holders the difference is fundamental and can't be fixed; for the RFH and grafmatics it can be remedied by changing the base plate. Occasionally you will find grafmatics that have made modified from the Graflex type to the Graflok type. Nowadays that is probably unnecessary since the Graflex type are uncommon and if you buy one by mistake it will be easy to sell on or exchange for the correct version with a Graflex SLR owner.
For more detail than you probably want, look here-> http://www.paulewins.com/old_site/re.../grafmatic.htm
Note that grafmatics will often slip under a spring back (but not always) while a roll film holder (RFH) will need to have the focussing panel unclipped and removed.