What is strap peg on the back of vintage Jazz bass headstocks all about?

it's a spare!

really I think it goes back to the days where people strapped electric guitars like classical ones

 

Never made sense to me either. I've never seen someone strap their bass that way, nor have I seen a vintage strap that was long enough for that application.

 

It is the way you used to attach straps way back in the day. Originally, guitars were more a rhythm instrument and were played in a sitting position. Country guys started to use banjo straps that tied around the neck behind the nut so they could stand. Early rock and rollers followed suit, so when Fender introduced it's stuff they were thinking along those lines as far as strap attachment.
Check out real early Elvis photos, or even Paul McCartney w/ the Hofner.

 

believe it or not i actually use mine sometimes. When im playing sitting down, its easier to just move the strap from the upper horn to the headstock than it is to adjust the length of the strap

 

It's called a hootenanny button if I recall correctly. You attach the strap there and play the bass in a position more akin to an upright.

 

Classic!

 

i was told it was an anomaly of the neck production process. something to to with the way they clamped and shaped the necks, it left a hole...so they stuck a strap button in there.

 

ive seen guys back in the day use a strap that was slung from the neck button thru the shoulder strap and then to the horn button as a remedy for neck dive.......cant say how well it worked but i know my mid sixties pre was really neck heavy

 

Fender made the three way strap for the 60's basses,
normal part attaches to the horn and skinny piece to
back of the headstock gives it perfect balance sitting
or standing,a friend still uses one on a 60's p-bass nostalgic.