I/O Box Seat; What hardware was used here

[abl1111]

I was cruising thru old SC pix ( looking at hardtops ) and came across this picture of a boat that had done what I was thinking of with regard to a quick install / remove seat mounted on the i/o box.

What was used as hardware mounted to the i/o box ? What mechanism lets this stay locked on place ?

I'd modify it to my liking but this seems perfect for my application.