Battery mounting

Hi all
Well I've come across an Item I'm not familiar with
It called the "battery tray (Brace)"
I came across this while looking for a battey "hold down"

DMPS-6636-BTB4 Mopar 1963-1976 A-Body Battery Tray Brace - dantesparts.com

I know my 74 does not have it, but can anyone tell me what it is suppose to do
and how do you install it..

Best