My notes (for an R65 frame I think) say 131mm between bearing seats.
EDIT: full notes:
The BMW head tube is 131mm between bearing seats, putting the inner races of the steering head bearings that same distance apart. The EXC stem allows the inner races of the KTM bearings to be 151mm apart, but the 7mm shorter stem (59401033000) from the smaller bikes should reduce that to 144. Machining the stem from 29mm down to 28mm lets the bearing drop down another 10mm, to 134. A spacer can take care of the rest, although that would raise the front of the bike. Furthermore, if the tubes are raised fully in the lower triple, the upper triple will not go down the extra 7mm, so we'd be gaining 7mm of height.