I was told many years ago that it was there to stop the bench seat from being adjusted too far forward. Seems that it would interfere with the shifter when shifting into second and fourth gear. I believe all four speed cars got this bracket regardless if bench, buckets or 4spd console.
My 72’ Demon has that - and still wears her factory rubber floor mat and 4 speed shifter. The low level bench seat is too high. I always thought it was for a center console.