Cleaning Up Some Found Metal

I've posted a few times about things my son and I have found while magnet fishing. This piece in particular I didn't really take great pictures of, because I thought it was basically just a round iron plate. It just happened to be in the background of a picture I took of some other pieces. Here's what I started with.

I started by scraping the surface with the back of a wire brush, just to see what happened. As I removed some of the debris, it became clear that there was some writing on it. So I kicked things up a notch and let some vinegar sit on the surface for an hour. I then used the bristle side of the wire brush to brush away what I could. Once again, a lot of debris came off.

I repeated this cycle several times, and here's what it looks like currently.

There's still some rust and debris to be removed, but I'm pretty pleased with how it cleaned up. Looking back at the original picture, you can clearly see the inner circle, so I should have known there was something hidden underneath.

I have no idea what the markings mean (other than the year, of course), so if anyone has any insights, I'd be interested to hear them.