You are correct, AL73 does not exist. I don't know what part of the game generates these. But the grids weren't all regular formed. Something with wrapping rectangles over a sphere.
As you can find in this Kriegsmarine grid quadrant calculator, it didn't find it either:
http://www.navalgrid.com/find/square/al73
And to prove what AL or AL7 looks like:
http://www.navalgrid.com/find/square/al and
http://www.navalgrid.com/find/square/al7
Not a regular 3 by 3 grid square by any means.
(In their About section there is in fact a screenshot of the actual map AL grids.)