Yep, definitely looks to be an older brass swivel retianer.
The brass swivel retainers were prone to damage from being over torqued and were also potentially able to be damaged if excessive stress or impact occurred to the swivel turret.
The older brass retainers require a more or less normal hex socket to remove although you have to grind off any taper on the end to ensure it has adequate bite. So it's basically impossible for a tech to miss that it is brass and not one of the newer stainless retainers that requires an allen wrench to remove or install.
Consequently, it would appear that during the last annual service it was either not removed (and the o-ring between the swivel cap and turret was not inspected/lubed/replaced or the tech serviced the o-ring and then re-installed the brass nretainer rather than replace it with a new SS retainer.
Either way, talk to your tech about it -although it is very likely he was not aware of the need to replace the retainer. Scubapro does not cover servicing their older regs in their tech seminars and while they used to offer a "vintage" class on techniques for older regs, I have not seen one offerred lately. Consequently, that type of information tends to get passed down in house from old techs to new ones and if the dealer is new or tech turn over has been high, that information exchange won't occur.
The last Mk 5 schematic (12/97) mentions the need to change the retainer in a service note on the page, but in practice these tend to be small hard to read print in the service manual. Plus the Mk 5 is so simple to service that there is nothing that prompts a tech to check the schematic for assembly assitance, to it is often skipped. And, it has been over 10 years since the change was made and it would not be uncommon for a tech to be generally unware of the need to change the swivel retainer and regularly used and serviced Mk 5's have usually been updated already so it is not always an issue.
SP also has a tech website, but they do a very poor job of keeping it updated with incredibly long delays before service bulletins are posted and again there are no reminders to inspect and check for needed updates on older regs when they come in.
As an aside, in the late 1970's and early 1980's Scubapro had a Mk 9. It was basically a Mk 10 but with a solid non swiveling swivel cap and turret for divers who did not trust swivel turrets. It was not very popular and is quite rare, as if properly updated and serviced, swivel turret separation is virtually unheard of.