So I'm 99% sure (until I develop a roll this evening) that you hit the nail on the head here!
Originally Posted by winger
I believe I may have been loading the film in incorrectly, and that means I could have five rolls of film at home that have been wound through the camera but not exposed! (I think I have consistently inserted it this same way, I didn't think it was incorrect because the start marker appears to be labelled on both sides of the paper!)
The 400H was actually loaded by the previous owner of the camera, and the shutter opens just fine without a film back as did this 400H roll develop properly in the lab.
I have attached images:
I developed a roll of Foma 200 this time to check if the film was different or not, there was a very distinct green tinted liquid washing out after the initial pre-develop bath in water!
IMG_2532.jpg: My second develop attempt (Foma 200)
IMG_2535.jpg: The mamiya guide
IMG_2535.jpg: Shows how I've loaded it to start with.
IMG_2536.jpg: Shows how I loaded it after more carefully examining the Mamiya guide.
Much the same results, very clear label markings.