I've wondered about this. It really seems like it wasn't a hard stop pivot for the shop. My ABZ is 2016 according to the serial number, but has 2x rectangle shells. I've seen other ABs that are supposedly older, that have rounded shells, and of course, just about everything newer has rounds as well. From what I can tell, it's somewhere around mid 2010s for ABs. You're suggesting it's 2008 for Zs, right?
Zs never had the square shells per se. (At least as far as I know) They had Super Fatties which were thinner. Like this:

