rumor include who might have put it there? wasn't the factory for sure .... only thing I've ever seen similar is a socket for the vintage style glass radio tubes, they have that shape center hole to align the position of the other pins, so I'd say probably some PO was in a trade that he used for a creative 'personalization'

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics