Why did the USB connector for the aluminium Apple keyboard have gaps on the plug? I used to own an Apple USB keyboard - this was the metal version that had a USB port on each end. From memory, the USB cable that connected the keyboard to a computer had several gaps in the connector itself. Out of interest, why was this the case? I recall that a USB extension cable was also included with the keyboard - I wasn't sure if it could be related to this in any way.
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.