Marking On Unfranked Cover Mystery

Enhanced, rotated 90º CCW. "POSTAGE" clearly spelled out in an arc at the bottom. "S" in center is obvious.

Here I go guessing again, but I think the cylinder dial reads 'Returned for Postage' and was hand stamped in the very spot meant for the stamp.

I just read the other recent responses on page one - John Becker sir I think you are exactly correct, it was most likely returned for postage and those are the clerks initials?

John, the backflap is sealed and the cover has been slit open at left.

Thank you.
That would point very strongly to this being an unpaid 1st class letter returned for postage and never remailed (or at least not in this envleope).
Upon further looking, I wonder if the scribble is a quick and messy "RTS"?

That's certainly a possibility. Good eye!

Not sure if this is related or not, but if it is, this cover is a gift that just keeps on giving...

A NY City banker named Augustus Dennis Shepard had a daughter who married a John Doull Miller, a textile importer/exporter, in 1891. The corner card on this cover reads 'Doull Miller & Co.' and is likely the same chap.
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The surprise? Augustus was a President of the American Bank Note Company!

https://www.webbhistory.org/PDF/09-Shepard-pt1.pdf

Held for postage seems very plausible. Key hint was Detroit to NYC. "Returned for postage" practical if caught in Detroit. Evidently uncaught as unpaid until reaching Station S, NYC - so held for postage notice given to addressee.
That just leaves matter of postage paid if addressee was sent a notice, since we don't see any. While it would be a rare situation, can we not rule out a "top of stack" situation as occurs often for postage due? That is, it only takes 1 other letter in same case at same time for addressee, and they (or their secretary?) came to Station S to pay the postage in course of other mail business and so on. And their payment (in stamps?) did not happen to make it to this second letter.

There is no evidence (i.e., a backstamp) that this cover ever left New York City.
With a return address, it was easily and quickly returned to the sender for postage, and it was never remailed.
The other scenarios are just extremely slim.