Black vs Red Oxide??

[YENKO DEUCE REGISTRY]

I have a '69 Nova that was black underneath, but red oxide on the firewall. The more black I sand off the firewall looking for 'codes' the more red oxide I find. This is the first I've seen this - not one bit of red oxide on the underside, but completely red on the firewall.

This is so strange that I'm not convinced that it's factory, so I've got a call into a prior owner to see if they ever dusted a few coats of red & black onto the firewall.