wanted to bump this based on something being discussed on the ESB facebook group.  Looks like the debut of Yoda on ESB 32B back contained a factory code in some instances, the letter U on the left side of the cardback next to bossk.  just perusing ebay they seem to be much more rare than without the codes, but this should be proof that the factory codes did exist prior to the 41A backs.  here is a pic:

You beat me to it,  all very interesting.



edit and an 'S'   ( on a Vader- thanks Oscar )