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
John
PS Owners manual for the camper. Popular question. See here
http://www.sunlineclub.com/forums/f1...350-11517.html
__________________
__________________
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club