Yes, that 121mm does look like the spoke hole circle diameter. Boy, that guy has some strange ideas about measuring stuff, lol. Wonder what he'd tell you if you asked him his shirt or pants size, lol.
the only measurement differences between the narrower hub, (Pics posted in post # 28), and the XS650 hub is the overall width, (including spacer and speed o drive), and the distance between the hub flanges. All other measurements are the same. for all intents and purposes the smaller hub will lace up a 36spoke 19' rim and takes XS650 rotors.
Center to center. to get a center line measurement there is 3 ways to do it.......
.if things are equal, flange thickness, (1/2 a mill won't make a difference here)............
measure...... over and under, from the outside of one side to the inside of the other = Center to center
under and over, same difference except measuring from inside of one side to the outside of the other. = Center to center.
Do a true mark center to center measurement, measuring from the center of one flange to the center of the other flange
There is no other way to do, sorry 5T our statement is arrogant dismissive and judgmental..................