Thanks to xenopeek for suggesting "inxi -S" (with a capital S) to get version number; this works. Also the file /etc/linuxmint/info gives version information.
xenopeek also suggested:
Open Update Manager, click Refresh to update it and if you're not told to update mintupdate itself open the Edit menu and see if the option to upgrade to Linux Mint 18.1 is there.
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An option to upgrade to Linux Mint 18.1 does indeed appear when opening Update Manager\Edit; apparently I'm still on 18.0.
Question: Ran "sudo aptitude update" followed by "sudo aptitude upgrade" on both 19-Jan and 25-Jan, and apparently this did not effect a change to 18.1. Is a different set of commands required?