Sku is an acronym. It is actually stock keeping unit and it is an inventory code to help control stock numbers.
It consists of a code number basically.
What it is in the sense you are asking is that the very early 12-a figures had the sku code printed on the footer. This only lasted a short time until the code was removed from the footer so if you have one then you have one of the earliest 12a figures released.
But not the earliest!
Before sku footers, the very very very early 12 backs had a white footer. Not many of them still exist, so these are very rare and desirable. So scarce they are thought to be salesmen samples for the release of the figure line.
The sku footers are quite scarce. I'm not bothered by them but some are. They do carry a premium.
Looking for: A fair priced ungraded trilogo Yak Face MOC (Condition is not so important - no cracked bubbles or broken seals though)