As has been said, the brass seats seem to have been lubricated and
sealed with tallow.  Nowadays I use silicone grease which is more
stable, but beeswax polish works fine.

If the threads are very tight it's probably because the holes have
dried out and shrunk with the seats removed.  Curiously, this could be
because the box has become either too desiccated or too damp!

Now, how to adjust correctly the electric and the pedal theme primaries
-- yes, there are 4 on a British pedal-electric Duo-Art -- is another
matter...

Patrick Handscombe
Wivenhoe, Essex, UK