Sp no longer makes them and they are different than the part used on the Mk 20 and 25.
Option 2 would be to find a dealer that still has one in stock, either new old stock or used in serviceable condition. More recent Sp dealers will not have old parts sitting around, but dealers who have been around for 20-30 years will most likely either have some Mk V parts still sitting around (they were made through the mid to 1980's and would have been commonly seen for annual service and upgrades into the mid to late 1990's) either on the shelf or in a box of old regulators retained for parts donation.
Off the top of my head,ahe Skin and Scuba in Pierre SD is one possibility. They changed owners a couple years ago, but the shop itself has been around since the early 1970's as an SP dealer and Tom, the old owner, was known for servicing and still having parts for older SP regs. Their number is 605 224-4154. They will probably not send you one unless you are a tech, but they would send one to your shop to install. The swivel retainer nut on the Mk 5 is a pain to change anyway as you have to modify a socket to properly torque it.
Option 3 be to find a used Mk V on ebay - either the original Sp version or one of the many clones sold by Tusa, Oceanic, and several other companies over the years. The Mk 5 is one of the most cloned regs in history.
