the Hand Extinguisher in the pic is manual – definately not automatic – and was the standard CO2 type fitted to all cabins and cockpits for dealing with internal fires.
All automatic Graviners had a “Banjo-Bolt” type of connection at their tops or bottoms, depending on the type. Some had multiple release heads/mechanisms. There was no localised capability for re-charging them and, basically, they had to go back to graviner-kidde for any maintenance. Many were pressurised – some up to 3000psi. They were nearly all explosively activated by Sqibs to break the bottle seals.
As potentially explosive vessels containing toxic/corrosive materials all the automatic types were removed (or should have been) when aircraft were placed in long-term storage, museums, etc, for return to the stock or to the OEM.
P.S. I dont know what an Mk21 looked like, but maybe graviner-kidde (or whoever that is now) could help you?
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