What is under the hood?

[whatleyr]

Hello. I have a 1965 chevy c10 fleetside which is very special to me. It was my grandfather's truck which he ordered brand new. it has the straight 6 with overdrive. Never been restored or touched.

My question: there are markings on the firewall in the engine compartment. I have no idea what they mean. I'm guessing they are factory markings when the truck was being built. Any thoughts or advice on what to do with them would be appreciated.

Pictures attached.