It is the API level ntg6/081 version of the software.
Based on what I could tell from the navigation, the original date was 2019, and the most recent upgrade was in 2019.
In addition to posting here, I messaged Mercedes, and they responded with a really useful response. They claim that future over-the-air updates will not occur if there have been problems with earlier ones. Next time, the update should be automatic, but for now, I'll have to manually apply it.
I now have a complete upgrade on a USB stick, and the instructions were really detailed, which is unusual these days. My next step is to locate an adaptor that will allow the USB stick to connect to the USB c port in my vehicle. Finally, I'm almost there.
By the way, my car doesn't have an SD card.