It's very interesting that this has come up, because Paddy and I were only discussing it with Nigel Silcock (Mr. Brocock) last Sunday.
It turns out that his late Father designed them, who sadly passed away and took his secret to the grave - nothing on the assembly procedure was ever written down, apparently, and was all in the old boy's head.
The problem seems to be that each capsule houses about fifteen washers of differing shapes, thicknesses and sizes. Nigel still has thousands of parts for the air capsules in stock, but without knowing where each indiividual washer goes, cannot reproduce his Dad's original design, even though he'd love to. Obviously, the permutations of different washer configurations when you have so many in each cartridge runs into many thousands. He and his team has tried to reproduce a working version from the parts they have, without success.
So, if you have one or more of these, they are a rarity and I suggest you hang on to them.
The good news is (maybe) that we have hopefully put Nigel in touch with someone who may be able to help. I've seen an example of this chap's engineering work and quite frankly, it's mind-bogglingly good, so watch this space.