A rapid is to air rifles what a Harley Davidson is to motorcycles.
People like them because they are simple, honest, relatively easy to maintain & do their job very well indeed.
They also just "feel right".
mk1 does not necessarily mean it's old they were made alongside the mk2, but if it is an early one it may have different thread sizes to the later models.
I'm pretty sure it's this thread size issue that requires the custom bottle/regulator adaptor, if it's a later model the block is generic to all models so any reg should fit, should you decide to fit one, as said above the owners club forum has every possible piece of information you could ever need or want both as documents or direct from people who know everything there is to know & the experts who fettle them.
The down side to Rapids is that because they are relatively simple people tend to play/tinker with them, untouched examples are rare & swapping a few parts can make one model look like any other, and some people will fit FAC parts to a sub 12 which might make it worse not better
Because of this playing/tinkering/customising value can go from £400-£650+ a good standard mk1 will be in the middle depending on calibre.