The M builds are in the same list as nightlies but in the column for type, they are labeled snapshot instead of nightly. I've read that it is not a good idea to flash M over nightlies. I run the nightlies without any issue.
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Do you think it's OK to put a newer SNAPSHOT on top of an older nightly, then? I'm good with staying with 12-12 nightly, and when M13 comes out, I'd want to put M13 on.