Took battery out. No rust on tray or in HH, but this area is clearly a mess. It would seem that tray/HH is coated with something (POR 50?). Could this have been a preventative measure by the PO...or is that just grime (specifically in the HH - tray is obviously coated with something)?

But why the corrosion/total mess on the sidewall? Battery acid leak? That's the only thing I can think of to cause paint to be flaked & surface corrosion up there. Or is this just what they look like after 46 years?

Under the car is rock solid - no rust under HH. What do you think? Thanks.

Looks like rust to me. I would remove the ECU to get a good look down below where the rust often does more damage.

Almost looks like someone put a fiberglass "patch" over the rusted tray.

Judging from the rusted side wall, i'd be digging deeper, i'm afraid you'll find more Fe2O3•32H2O

Looks very typical for battery acid spillage in that area.

Pay attention to those two white clipped wires passing though the sheetmetal. When folks just drill holes though panels like that it has potential to become a hot spot for rust to start.

As stated by SirAndy, fiberglass quick fixes are a pretty common plague upon 914's by those that didn't want to be bothered to do metal work.

I can't tell from pictures but someone definatley put something (POR, Undercoating, rust neautralizer, black paint, or orther crap??) over the battery tray and shelf area that isn't an OEM coating.

Looks a little like a rust sealant. Two of the repair options are sealant or a converter. Third being a replacement. Take a magnet to it for a fiberglass test.