Bingo!
Our man from Plymouth has it almost to a tee.
Actually, I acquired a couple of print drums to try reversal processing of paper negatives (because I didn't want to handle Sulfuric acid and Sodium metabisulfite in open trays in my cramped unventilated bathroom-cum-darkroom).
This device is from a Paterson 10x8 colour print drum, and is a spacer that allows you to process 5x7 or smaller in the same drum (the spacer sits vertically in the tank, the paper each side of the pegs, the latter allowing developer to flow when the drum is rotated. The spacer is removed of course when you are processing 10x8s
I've been looking at the damn thing for several weeks wondering how on Earth it was supposed to work, until the penny suddenly dropped with a clang a couple of days ago.
I haven't tried it yet with 10x8 film negatives, and for 5x4 film I use a "taco" method where you curl them in two with an elastic hairband round and develop them in a Paterson Universal (no reels, just the centre column)
The Sulfuric acid/Potassium permanganate bleach, by the way, is pretty hard on the the emulsion layers, and it turns out that drum agitation added a level of mechanical stress to that, so the moment I got the print into the second developer, the emulsion floated straight off the base ... so I've had to start using open trays anyway :sigh: