Most has already been said here & by me in similar old threads.
There is nothing wrong with a 1987 Stainless Steel Water unless:
a) It has a dent in the cylinder (or a deep gouge), or similar damage to the threads, etc
b) The stainless has been scratched and corrosion has taken place
c) The unit cannot retain pressure due to a valve leak in the head (or the head/gauge is impact damaged) and either a new head/valve is no longer available (e.g. Abtex) or would end up costing more than a new extinguisher.
In other words it has become unsafe for use or un-servicable due to lack of parts. Just needs annual Basic Services & 5 yearly Extended Services like anything else.
Stainless steels were preferred by some not just for appearance, but for long life & durability, hence the MoD using Chubb S/S waters, powders & foams for over 20 years after stocks of the 70's/early 80's Rampart (also chubb) cartridge & chemical stuff wore out.
It wasn't one of the same group of companies that likes putting little square yellow 'corrective action required' stickers on extinguishers left, right & centre was it (I liked the way they used easy peel adhesive on them - makes my job of ripping them off again far simpler!)