Are all SP1 forks equipped with the little "reservoir" in front? Reason being, I found some builds on japanese web, they didn't build the RC51 but the bike I own, XX, but I think they used SP1 forks which is somewhat "common" fork upgrade on the Bird. The problem is that aftermarket WP shock makes it harder to identify this, and also they use 6 pots, so they must use some adapters for this right, OEM is 4?
Yes all SP1, and SP2 OEM showa forks have the compression adjusters mounted on fork bottoms. Aftermarket forks like the WP pictured, or Ohlins forks do not have the adjusters there. SP1, and SP2 came with Nissin 4 piston front calipers, you can get 90mm spacing brackets to use 6 piston calipers. Can also get Aftermarket radial fork bottoms.
SP W was a designation for the race bike. SP Y is a 2000 model year in the UK. RC51.org is a good place for information.
From what I've gathered, that Y thing is prominent, but only in the UK. Haven't heard the term SPY used from any other countries that I can recall. It has to do with the Y being the VIN designation for the year 2000. Otherwise we just go by SP1 which can mean either 2000 or 2001.