Today I have been looking at the trimmer capacitors that I bought at the Eastbourne Radio Rally. The bags of "10 for £2" are all marked with their values, which is good. I did check them with a capacitance meter, and they measured close to what was indicated.
Need to build some nice tuned circuits now!
I like the miniature air-spaced ones, they have a certain old-world charm, and presumably give quite high Q factor tuned circuits. The vanes are vulnerable to getting bent, but you can see clearly how much the vanes are meshed and estimate the capacitance from that. The ceramic ones are harder to "read".
I bought two of the "piston" style trimmers. They looks to be very low value pF, and can't really be measured on the LCR meter. I guess they are about 1 to 5pF, but I can't find anything quite like them on the internet. The threaded screw is wound down into the ceramic tube to adjust the value. I think I have seen similar components on UHF TV tuners in the days of analogue TV.