Sorry, it was an update for build 1901 for the stock MM machines. Somehow one of my registered beta machines was mistakenly targeted for this OTA update.
Labeled as 'bug fixes' and 424.5 MB size
July 2017 security patch
I dont have a box running MM so I can't speak to any differences.
My beta device has the same update available but it wouldn't go through with installation. It downloads but then gives an error after booting into recovery to install.