Al,
Clearly it was modified since the 6C4 wasn't used until the HRO-7 and the earlier "double ended" RF tubes were changed to the 6SG7 single ended type. One of your first decisions will be whether to return it to stock which is always my bias, it depends upon how hacked up it is from tube type changes.
I agree with Roger. BTW, if a pre-WWII HRO uses the 6C4 in the HFO, it was probably modified after Loren Windom, W8GZ, published his article titled "Modernizing the Prewar HRO," and appeared in the June, 1942 issue of QST. In addition to the 6C4 mod, he discusses comparisons with the HRO-7 and shows how to add an OA2 VT tube -- just like the HRO-7. The 6C4 and OA2 were added together on a small chassis plate. Link below if you're a League member:
http://p1k.arrl.org/pubs_archive/29312Depending on the similarity between the mods, you may be able to follow Windom's diagram and reverse-engineer the HFO back to the HRO's original condition -- or at least understand what's happening in the event you decide to keep it as-is.
Paul, W9AC