Oh,
I forgot to add,
you can easily tell if you have the sure brake option.
look under the hood just to the right of the radiator (passenger side)
and if there's a complex looking canister device that looks like this
illustration:
http://imperialclub.com/YearbyYear/1973/FSM/Chassis/html/05-080.htm
It's the presure regulator for the rear brakes.
you have surebrake,
I think all instrument clusters have this window, it's just not lit if they
don't have the option.
Jack