I don't know of any makes that stamp width in cms other than stubben 29 is slightly on the narrower side of medium (they range from 27 (narrow) to 32 (wide)
Its an English Keith Bryan saddle. I'm surprised too as I thought he stamped NMW on his saddles. I wonder if the seller is confusing it with the serial number....
As the measurement is taken across the tree points, the length of them means that a given cm measurement could be a variety of actual angles ie width fittings depending on brand and model. Passier use cm as do Prestige but i believe both come up much wider than Stubben, as the latter has longer points.
I would call Keith, you may be right about it being part of the ID, or even a stamp indicating which tree it is.