Cincinnati Milacron Eames Chair – Dating the logo  

I can only add that the shells with the embossed HERMAN MILLER letters began production in the United States in or around 1960. So this shell is 1960 or later.
If there are imprints in the shock mounts that indicate a standard H base, it was probably an H base with white plastic glides.
Hard to tell, but IF that is the residue of a white paper patent number label under the letters in the photo, then it is probably the later (larger) paper label. (Slightly smaller paper patent labels were on shells from 1954-55-56ish) If that label is some other shape than the rectangular paper labels with rounded corners, then I have no clue what it might be.
Generally, I am a little light on my understanding of Milacron shell inclusive dates. And I am not much of a help on when the change you are asking about happened, but the embossed letters clearly started on both Summit and Milacron shells in about 1960, and appear on just about all US production shells right through to the late 80s or early 90s.
Hope this helps.

Thanks for the quick answer!
The residue definitely is from one of the large patent labels and the shell has a wide shock mount configuration. The imprints on the shocks are from a stacking base but I got it with a later-produced lounge height H-Base. So no real clue there.
I'll go with early '60s then. 🙂