16:1 manual steering box to trade

If that 16:1 box's worm gear is shot, then the trade is useless to whomever gets that box since the 16:1 worm isn't available anymore. If your box needs rebuilding, why not simply get the 20:1 worm from Mancini and do it? At least that way, there's no chance of another member ending up with something useless.