They were held down by springs. Tight coiled springs (2) in either an "X", or in parallel to keep them strapped to the floor board. That's the only way I've ever seen them.
In fact my '64 has them too! smile

HTH

Chris

EDIT: OH wait. I just remembered. I've also seen a canvas apron across the back of the bench seat with pockets for everything. RPO? dunno. smile

Last edited by Detroit Iron; 09/03/2019 3:13 PM.
'64 Chev C20 LWB stepside (Ol'Blue)