I've finally come to be able to see the differences but can't say with certainty which bezel is what generation.
- The shape of the "S" in "UNITS" - can be sharply angular like a "Z" (older), or softly serpentine (newer)
- The alignment of the verticals in "N" and "I" of "Units Per" over the "H" of "Hour" - Oldest 1st gen has the "I" just a nudge past right vertical of "H"
- The shape of the "4" in "140" - with the older being the one with the more flat-topped trapezoidal shape; more acute top angle in newer
- The shape of the "2" in "120" - whether the descending slope intersects the horizontal foot, or drops into it at via a vertical that joins at a 90-degree angle (older)
- Differences about how the dot over "100" relates to the last "0" and how close to the inner edge of the bezel
- Between the 1st and 4th generation the zeroes became rounder, as if more inflated
To my eye, the middle one is the most recent, post-production 4th gen.
Top and bottom are either 1st or 3rd gen. If I had to choose, I'd say the top is the oldest.
Hopefully someone else (who really knows what they're talking about!) will contribute!