The Alpha catalog, like Alps, Noble, etc, is published so you can make up an order number. They make them custom. They make a few generic ones for sale by parts houses, but companies like Fender or Peavey order them custom.
What is it for? And description? Round/square? 16mm, 12mm, 24mm? Shaft type? 50k...linear, audio, reverse audio, other taper?
Why does it have to be from Alpha? They all make 16mm split shaft 50k pots.