Well, it's not looking good. Not much of anything is showing up. It looks like there is a yellow line by the fill plug.(did they use paint or a grease pencil?)

There's an orange dot about the size of a quarter with a run on the top and remnants of a stencil on the side. I can't make it out though.

I assume the muratic acid will just eat everything away right? Is dipping the entire housing in EvapoRust the only option to remove some rust without destroying the marks? I soaked the original coil in EvapoRust and all the marks reapeared.

Thanks for the suggestions so far.