Royal X pan had a speed between 800 and 1200 IIRC. I have given away my last batch, so I cannot remember. I can say that it was very foggy and grainy and kept very poorly even if frozen.
Oh I know it will be foggy, grainy, and down right miserable. However, I was using it to shoot some statuary, so it might give me an effect that will work. I also used a couple rolls of fresh Delta 400 / Acros 100 so I have my backup. This was solely an experiment.
OK, you basically have the same developer. And loaded with sulfite and metol which will
bring out as much spped the Royal X has remaining, but also cause the most fog as you can get.
Try HC-110, or a Phenidone developer without much sulfite. Or a dilute print developer.
Monobath with HC-110!!! this is the perfect situation for this albeit I have never tried it. But the low-fog properties of fixer while your developing this old film should counter-act it!
Well, it is done now. There is a perceptible image, but there is two metric tonnes of fog. I am going to go to the darkroom and see if I can pull an image from it, even if I have to go high key to get one. Results to follow