[blucar]

The early cars do not have knock-outs in the firewall, however, in some entrances there is a slight dimple where a hole could be drilled for an accessory.. I believe the radio receiver box was one of those dimples.
Many of the comments in this thread have failed to mention that the pre-40 cars have floor shifts, therefore the location of the heater should be on the passenger side in lieu of the center where it will conflict with the gear shift.
The comments in my first posting to this topic (#2 ) made mention about the '39-40 heater in my '36 coupe. As I have stated, I believe the indentations/holes in the firewall insulation are very similar to the mounting in my car.