Why did the USB connector for the aluminium Apple keyboard have gaps on the plug?
If you are referring to the groove in one of the sides of the USB plug on the end of any wired Apple keyboard, it was designed to mate with the included USB extension cable. This extension cable was constructed with a guide inside the female end which allowed only the insertion of the cable from the keyboard -- it could not be used as a general USB extension. The male end of the extension cable was without a groove, so stringing two extension cables together was not possible, either. Thanks, Apple. 

Keyboard USB connector:



And yes, I need to clean the cruft from my keyboard. I've been WFH for many hours a day for a few months and there just isn't time.