This is the grille of the Duo-Therm that I've seen mounted "stock" in the 1970s Trilliums. Bill, is this what your furnace
looks like? I do also have copies of the manuals for this model (and so could get a model number if you don't have it).
(Of course it could have been added later and be different, but since I have a photo of the typical stock Trillium
Duo-Therm showing the grille I thought I'd post it.)