H'mm That does not look like a Hydroflame unit but I have not seen a lot of the really older ones.
It might be a Suburban unit.
Can you take a few more pics? A tell tale is the outside grill area on the newer ones. Suburbans and Hydroflame really differ in this area plus a lot of internals.
The 1st part of figuring out the missing part is to figure out what make and model.
TerraLab Engineering may have been some kind of 3rd party testing place or they use to make furnaces long ago.
Hope this helps
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