I just learned from my dealer, who got it from Garth at MS, that the P-to-S distance is 292 mm for the 12-in. wand and the counterweight is the same. The range of cartridges shrinks from 4-18 grams to 4-16.
Apparently, Bob Graham is on vacation until June 9, at which time Garth will ask him for a cheat-sheet with comparative data for all armwands.