I had the exact same one, only also the complete unit.
(Note that there are small 'button feet' in pix 1.)
My unit was double walled cast iron without a bottom but it has 3/4" pipe thread inlet and outlets on the backside.
Also only had a door and no removable top.
It was a hot water heater!---possibly meant to be installed outside or on an earthen floor due to the lack of feet.
The other interesting thing is that there was no means to attach the firebox to the base, it simply sat on the base!