Like a lot of things, these were stripped and painted many years ago.
Wisely I decided to test them before fitting. I applied the air gun to the hole and nothing much happened. I tested the next one and a small movement was observed. I squirted in a bit of WD40 and tested it again. The vapourised WD40 covered most of the desk now - but both pistons are moving.
On the first caliper, this method only moved one
Fitting the calipers was fairly easy - but the Disc dust shields needed a bit of a tweak to allow the calipers to fit.