As said above take the gas strut apart ( relatively simple) & you’ll find all the info you need printed on the old gas strut.
Don’t use Selden to source the spare. Lots of suppliers at gas struts online.
If you cannot find the spec, I had the same issue when replacing my original spring with a gas strut, got a fully stainless strut with adjustable gas valve, fitted it and tried it and let some gas out until it was correct, was not difficult to do. The manufactures name I cannot remember but recall that they were in Derby ? They supply the seldon struts from what I was told but direct from them were fraction of the price, I had to make end plates but you should have them from the old