My 4x5 Series D curtain just gave in to old age. Come to think of it, the number of slits should be the same in both Series B, Series D, Super D and Speed Graphic? I haven't compared, but could it be that the curtains are interchangeable save for the tension shaft? Could I glue a Pacemaker Speed Graphic curtain to the Series D shafts?

According to Morgan and Lester's Graphic Graflex Photography, the shutter curtain on the Graflex SLRs is slightly longer than that on the Speed Graphic. It might be possible with some improvising to adapt a Graphic curtain, but I'm not sure about that.

RB Series B, RB Series D, curtains of same film size would work. A curtain for a non-RB model or Graphic won't. It's not long enough to cover the vertical dimension of the format (5 inches tall). 4x5 Super D had the 'simplified' shutter, may not interchange. Ask Bert Saunders, he'll know what will work.