I can't be sure ,not having had a "50" but at that time slimlines were often fitted with a Ewarts single plunger tap that had a double action ,pull on,then for reserve turn and pull again. These had a longer plunger than is common (and a longer body) ,the plunger has an extended brass forked nose,hard to find ,takes a normal cork. No one (except me!) has correct new plungers ,I bought a box full!. So I am allright Jack. Some have an extended pull with an internal thread and nut, I have helped a couple of Norton nuts out in the past.