I ran across this at an upcoming auction. It’s the first time I’ve seen a coin simply listed as AU. (other than a details coin). Is this an early version of Details Grade? These might be common, just the first I’ve seen. I checked NGC site, but did not find the era in which it was slabbed.
Not very old, but it is unusual. I've seen several like this. No idea why the person sending it in wanted them like this instead of an actual grade.
Most like these are some sort of bulk orders from dealers. It probably costs very little (maybe as low as a few dollars each) and is worth it for them to guarantee the coin is genuine for buyers.
Some of the bulk submissions allow the person to select a minimum grade (say AU 58) and everything lower than that would just go into a genuine holder (like the one pictured in the first post).