Gorbas: I can see a bolt supporting a core there - is that where your source spool goes? Does anything else support the cassette spool or do you just hand-hold that?
I'm loading cassettes into the back in the daylight and assuming the felt traps work OK. The Mamiya back requires you to wind on two frames after closing the back before it reaches "1" on the counter, so that will pull enough fresh film from the spool. While I could no doubt load in the dark and get two more frames, I don't know the back well enough to do that yet and it's fiddlier than a 120 back. The whole bottom comes off in order to insert the cassettes, and it has to go back on quite precisely.