The firmware as printed on the nav is DIFFERENT that the operating system SW. (And this is all different than the upgrade versions V24, 25 etc)
For Sirius you need Nav firmware 3.0 or 4.0. either will upgrade to the latest operating system.
(I was concerned that my Mark 4 which was firmware 3.0 would not work- it works fine.)
Note that in the SW version, (ie the SW4-1/42) the first four signifies Mark 4 (IV)...
You've prolly seen this but
http://www.x5world.com/articles.php?...ticle&artid=59 is a good discussion of upgrades disk versions and the resulting operating system versions.
A