...lack the SX stamp on the baseplate under the balance wheel.

Picture borrowed from the Cal 11/12 article, so the I (this movement is a Cal 11-I) is highlighted, but it's actually the SX stamp to the left I'm interested in for now.

I wonder what the significance of the SX marking is? Perhaps a sign of a series production movement, but it doesn't seem to appear on all watches that are otherwise standard, so I'm not sure its absence implies a prototype. But I do know that none of the 28.8k Cal 11s I've seen (these 3 and the one in Sandra's Autavia) have had this mark.