Stuart Tallack contacted me as follows:
Out of curiosity, I bought the Optochrom tank that was on eBay recently. It is in quite good condition with a well executed and robust repair to the lid. The drain plug was missing which was a little awkward as it is a moulded thread and the female thread it goes into is difficult to measure. A cork was not an option as it needs a hole recessed into to it to accommodate a metal part. I cut a fairly sloppy 8TPI in a stub of hard vulcanised rubber and used a soft plastic grommet to seal it. It holds water without a leak so will do until I grind up a better tool.
The tank came with some supplementary instructions but not the full ones. If anyone has a copy of the instruction book, I would love to see it. There are brass female threaded parts moulded in to the Bakelite to accommodate the screwed spindles which hold the spool in place. As seems common at the time, the threads are needlessly fine and the male ones are quite a sloppy fit. They would be very easy to cross thread. How the tank compares with the Agfa one depends on the ease with which the film threads on the spiral. That remains to be seen. I imagine that it would, like the Agfa tank, dislike the thicker films. I’ll get round to it soon and if it works reasonably well I will make a better drain plug.
My supplementary comment was wrong, though; the drain plug does need a recess in it to accommodate the end of the little spigot that the reel sits on. I thought for a while that the spigots went the other way round but I was wrong. What I really need to know is how it is supposed to be agitated. The reel can be rotated back and forth through ninety degrees and, of course, the whole tank can be turned or shaken. It is an ingenious tank but I am a follower of the less-is-more school. It is possibly too clever for its own good.