Have a look here
SCUBA products TX40 to TX50 conversion my brother in law got a bunch of adjustment knobs made up in stainless steel. These will replace broken ones or upgrade a tx40 to a tx50 (atx40 to atx50s too).
I am just testing this lot in the sea (initial results are favourable). Will be for sale soon probably looking at GBP10 each -less if you buy more.
Incidently we are just about to make the next lot of stainless steel dinplugs and delrin dustcaps - prices will probably rise so you should order now at the old low price
Best, Graham