The initial pay is actually “Fleet Reserve Retainer” pay, until you hit, IIRC, 60. THEN you shift to the Retired List.
(oops, making a point already made)
Fawlty Towers was representing a common reality, that a hotel like that would very likely have had a retired army officer with no family living out their years. It used to be said you could put your head round the door of almost any pub and ask “Has the Major been in?”, and it would make sense.
Compare and contrast, for example, the David Niven character in Terence Rattigan’s Separate Tables.
Yeah, the Navy and the Marine Corps have that unique construct called the Fleet Reserve that neither the Army nor the Air Force have. You’re correct that it’s actually called retainer pay until the member hits 30 years of service (not 60 years old), then they are automatically transferred to the retired list.
The fleet reserve and retired reserve are really a distinction without a difference. There have been discussions to eliminate it, but it’s really not worth the effort to change it.