I'm curious about what the circular purple marking might be.
They are clerk or carrier initials.
Consider this card addressed to Marion, Indiana, with the initials "C.P.M." in a circle, run against the employee list in the 1909 "Official Register" = Clie P. Maffet
Again, although not boxed, "Cased by C.R.C." run against the 1911 "Official Register" = C. R. Cooke
And lastly, "A.C.J." run against the 1911 "Official Register" = Arthur C. Jones.
This card also has a wonderful list of city and rural carrier numbers. I do not see any specific reference on handling these in the Postal Laws & Regulations volumes, but I strongly suspect that mail for which the clerks cannot identify the carrier was stamped with the numerical list and left on a common table for each carrier to examine daily. In this case, all carriers checking off within one day, the card ended up at the general delivery unit (where the clerk forgot to change the date in the dater and it is one day behind the machine cancel!), and eventually clerk Jones figured out to forward the card to North Manchester.
The cover in the original post has the initials of a New York city clerk, of which there are many hundreds. A clearer scan of just that mark might help ID the clerk.
(The "Official Register" is more exactly "Official Register, persons in the civil, military, and naval service of the United States, and list of vessels, 1909, Volume II, The Postal Service", issued in odd-numbered years. Volume II covered the post office and ran only through 1911 before being discontinued.)