As long as evaporust doesn't mess with plastic or copper you should be OK. I found once you get some noise a good squirt of penetrating oil or WD40 into the adjuster screw hole and stuffing a rag in the end and running it for a while off a battery usually frees them up.
The date codes are stamped into the long edge of the outlet on the face of the scroll. Looking at the scroll side, along the long side of the opening they are usually stamped just below the lip. Many are very faint, I'd say half I've seen are readable to some extent, few with full crisp stamps...
I don't have a photo but if you want one I may have a set in the shed that are blasted and can take a snap or two.