Marking On Unfranked Cover Mystery

The 1898 cover shown, given the completeness of the machine cancel killer's wavy lines, appears never to have been franked by a postage stamp (ie there's no indication of a stamp having been removed), yet there is also no indication that this cover wasn't delivered (due).
There is a faint, circular, magenta marking at top right where one would expect a stamp to be affixed that I think reads 'Postage' at the top, inverted and running to the 9pm position (so this may simply be a case of set off), and a number at the center (that I can't make out, even using an image program).
Anyone recognize or have any idea of this marking's relevance, if there is one (Held for Postage), or otherwise how this cover might have gone-thru the mails un-franked?
Verso is blank without any markings.

It appears to be a near albino PSE with the faint postage printing accepted as the postage it is..

Interesting thought, but keep in mind that the wording is inverted. Is that a possible printing outcome in a GSE? I've seen albinos before but have never seen an indicium printed backwards. I just don't know...

I've seen envelopes inside out

But for those of you who see this as a possible albino ... What die has an upper-cased "POSTAGE" arced this way? I don't see any.

Doc, is there *any* hint of embossing?

Good point about the embossing, John, and no, there's no hint of any on either side of the envelope.
Your post also gave me another thought: This cover is postmarked 1898 and as far as I can tell, there were no GSE's issued with circular indicia until 1903, beginning with Scott U379 (and these were issued in green, not magenta). Clearly, this is a handstamp (or its set-off) and not some sort of GSE EFO.

I'm very much a newbie here but is it OK I take a guess and maybe say that could this magenta handstamp possibly be a Free Frank NY postmaster general 'postage paid' handstamp? And is that the postmasters initials in the upper right corner? I'm just guessing here since the legible cancellation says it was cancelled at the New York, NY P.O. Branch 'S'. There is a big 'S' in the middle of the circle.

Perhaps the Doull Miller & Co. mailer knew the Branch 'S' postmaster personally and asked to have this item mailed without paying postage? Could that have happened back then? I don't know, maybe I'm creating a story where there is no story.

Newbie-schmoobie. That's a good an explanation as any so far! Welcome to the forum...you'll find this to be one of the best philatelic mb's available!

There is a zero chance of any postal employee doing some sort of free frank for a person or business.

I suspect this may be a case where a clerk was fooled by the faint imprint resembling a government envelope - thinking it was an albino or almost-albino and passing it.

And sometimes, things just slip through the system.

I agree, there appears to be an "S" in the center, on its side. It would be interesting to try adjusting the contrast, brightness, exposure to some extreme amount to see if it can be made more readable.

docgfd, thank you so much for the kind words and the warm welcome.

John Becker, I figured as much but wanted to take a guess, thank you always for your knowledge and insight

The 'S' at the center of the marking (which I believe it to be) matches the dial on the machine cancel: 'P.O. Branch "S,' seemingly indicating whatever it is was applied at that NYC post office.

A Frajola board member (Leonard Piszkiewiz) suggested this could be a 'Returned for Postage' marking, although I would think there would be some other indications that this was the case, albeit at this point, this appears to be the most likely explanation.

No amount of image manipulation (at least in my hands), including using 'ImageSleuth,' has brought out any more of the marking.

I like the "returned for postage" idea.
If it went all the way to Detroit, it should have a receiving mark in this era. A lack of such marking would also be consistent with it staying in NYC.
Or if 3rd class, the flap would be left unsealed/tucked. Either way, the importance of knowing about the back side no matter how mundane.
(It would also be nice to know from the beginning when items are cross posted to more than one stamp forum.)

The faint marking you're seeing could be an indication of that, though it's hard to tell without clearer details. As for how it went through the mail un-franked, maybe it was an exception, like a mistake or oversight at the time.

If possible, this should be examined under UV.

John, my original post mentioned that verso was blank (ie, no markings whatsoever). The cross-posting was several days apart and although I get your preference for having that noted, my rationale was along the lines of not wanting to create a pre-supposition to anyone's analysis. Today I figured it had been long enough, so I released the proverbial kitty from its bag.

Rev, good idea! I have an old UV lamp lying around here (old as in 40+ years LOL) so I'll dig it out to see if it still works. Dunno if its LW or SW...

I see your initial "verso" mention now, however, sorry to revisit the back side again, but..... this is why it is much easier to show it.
The remaining point is sealed vs unsealed.
Sealed would mean 1st class and high chance of returned to sender.
Unsealed would mean 3rd class and (not generally getting backstamped, some greater chance of actually being delivered.