The "International Service Markings" Machine Cancel Society Specialized Study No. 30, lists 2 machines each in New York City (1910-1913) and in Philadelphia (1921-1930). Here is one of each, the Philly one being the same as your except the dial is slightly different:
I do not see much in the "Postal Bulletins" except the mention that there are "letter boxes" and "combined letter and package boxes".
The "Postal Law & Regulation" volumes also mention depositing parcels: from 1924 (Section 521, parcels less than four ounces may be deposited in mail boxes) and from 1932 (Section 715, parcels of third class matter may be deposited in mail boxes).
Len Piszkiewicz mentions on page 545 of his monumental "Chicago Postal Markings and Postal History" the New York and Philly markings being placed on mail deposited into a lower class of mail, similar to the markings "found in parcel post" or "found in circulars". This seems to describe the situation nicely.
It would appear that NYC and Philadelphia had enough mailings of small parcels that they installed dedicated parcel boxes, but patrons dropped enough first class mail into these parcel boxes by mistake or perceived convenience, and that a canceling machine was found to be efficient for handling that mail. I suspect the parcel boxes were not emptied as often, and this cancel served as a notice of why a mail piece might be delayed - by being deposited in a slower class of mail.
It would be interesting to see more contemporary documentation of these. Quite collectible!
Add: Two covers found in lower class mail and marked:
1958, first class mail "found in circular matter ..."
1927, registered mail "found in ordinary mail", caught during processing at origin city of Indianapolis and backstamped, etc.
And conversely, mail found in a higher class such as "Not in air mail" typically found on mail with only first class postage but on an envelope marked airmail.