Excellent question!
The one in my '49 1-ton is an ABCD fire bottle (good for use on petroleum fires and electrical fires) and it's mounted where the fresh air heater would go if my truck had one. I just used the screw holes for baffle plate (that blanks out the opening for the fresh air heater) so I didn't have to drill any additional holes in my cab.
If you have a heater there then obviously you won't be able to put your fire bottle there.

John