Well, I only got mine in the post 15 hours ago, but I would say 'not without some modification'! The back snugly fits a standard double dark slide holder, but doesn't have the width for a grafmatic, or the graflok sliders for a roll film holder. You might be pushing it to even get one of those old Calumet or adapt-a-roll roll film holders in either (although having now said that, that's the first thing I'll test when I get home later on)...
I have used some black foam board from a craft shop, which when cut to the MPP back size will make up the thickness of a Toyo darkslide. Slip the together and you have a lightproof film holder.It cost me £6.00 for five sheets in a pack I only required a quarter of one sheet,but thats the way its sold...
Singer Graflex RH-50(67/70mm) is 121mm height. would it work? Since its not flat i would need foam yes. What are the dimensions of the harmanback for filmbacks?