Ok - it’s pretty simple.
Spooky are making 750 machines. It’s a number they are comfortable making. They don’t want people waiting 2 years to get their machine.
All 750 machines are the same playfield and gameplay.
If you want to become a Fang Club member, you get an earlier opportunity to buy one of the 750 machines. You also have the choice to upgrade your machine to a Fang Club Edition for $500 (ish) which will be the same game, with upgraded armour and maybe some other trinkets.
Spooky are clever enough not to sell 1000 Fang Club memberships - or the shit would hit the fan.
PS I have 9 people on the list for R&M in New Zealand. I can’t think of any other new pin that sold more than that down here, outside of Metallica and AC/DC.