I thought most stubbens width was a number from 32(extra wide) to 26 (very narrow) which is usually stamped on the buckle guard?
Mines a 29 which I think is medium
They have long points so not suitable for most W plus horses, the cm measurement is across the points, no idea what the size B is. Because the points are longer a 30cm Stubben is narrower than something like a 30cm Prestige where the points are shorter.
I think I'm right in saying that the 'B' bit is the width of the seat, (flat seat), being 'A' the narrowest to 'C'.
Double check on this website Stubben-Specialist...
The confusion is because the old trees had cm widths and the new trees have letters. Best bet is to find your saddle on the website or to fire off an email and ask their advice.