Well, wouldn't you know it, PSC does this just as I am determining that a new box may be needed as well.
But there is at least one other option. An outfit called Lee Manufacturing in Sun Valley, CA will rebuild these boxes. They will even refit them for 3-1/3 turns. But they have no cores, you have to send yours down. Turn around for them is about 5 days.
A basic rebuild was quoted as (ahem) dollars, which includes seals and o-rings (no hard parts). To convert a 4 turn to a 3 turn requires 2 hard parts for an additional (cough) dollars. If you do all that the total is still under the price of a new 3-turn box from PSC when they were available, but not by a whole lot.
Call Lee Manufacturing at 818-768-0371 for details.
-Jay