Yes, previously, I had found one of the underground valve covers under my lawn. The round detents on the cover were both perforated, which allowed water, dirt, whatever to get down to the valve & wiring area. I tried to find a replacement, but apparently mine was a bit obsolete, and the store and online covers that I found were all too big or small, or just suspiciously different. The retailer websites generally showed very little if any detail on the covers, which wasn't helpful at all.
@emil I didn't want to have to replace the whole undergound assembly, so with some more Google research, I found the exact replacement shown at Sprinkler Warehouse. Their website showed a detailed drawing complete with measurements, and I ordered three of them, since they were so cheap. They shipped exactly what was described on the site.
@emil The Southern California valves were all above ground. No wiring problems, just valve failures, mostly diaphragms as I recall. I didn't know underground valves existed until we retired and moved to cold country, in North Carolina. We have much, much lower mineral content in the water here (about 30 ppm), which, along with the protected underground placement, I suspect helps to explain the extended life of the valves.