If you are referring to the box with the tubes, mine was fastened by two bolts or slot head machine screws (stovebolts) through the firewall.

@tiny That's what I thought too but I don't see the bolts coming though the fire wall

Your first picture. About 3-4" directly below the V rod and the second is 8-10" directly left. Slot head screws.

Thank you, Tiny, I was looking for bolts instead of screw heads

I had a 1937 car with a radio and antenna under the running boards which I'm quite sure was an dealer installation from new.

The radio box was mounted with two studs through the firewall (in the location of the machine screw heads in your picture) and nuts on the engine side.  There were large external tooth washers under each nut.  If you can find radio installation instructions for dealers it should clarify the recommended mounting.

Tom

Thank you for the information, Tom   I have another box that has the bolts which I think would be easier to install

.   I'm going to check the one that is installed first before I attempt to remove it

I initially put all-thread like that on mine when I first put it in. Operating from memory now and it's been a number of years but someone (Gene I think) advised they were originally installed with slot head machine screws (stovebolts) so I removed the all-thread/nuts and replaced with machine screws.

Thank you, Tiny   I will use the slotted head machine screws when I puy things back together.  Is it a two-man job?  I don't have the radio in front of me now but is there a way to get to the tubes without pulling the box out?

I did it by myself. I just did one fastener at a time. It was already installed and held in place by the all-thread. I R&Rd one side then the other.