Help With Describing USA Incoming Cover - 1903 Baltimore Rpo

I am a postal history novice, especially when it comes to US covers. I recently acquired this cover sent from Jaffa, Palestine to Baltimore, MD then readdressed to Princeton, NJ.

So far this is what I can describe about the cover, but I am not sure about the chronology related receiving postmarks.

I am also not sure what the Baltimore postmark on the front of the cover is for, it has a large numeral 7 in the middle and the year 1903.

Austria Levant 1900 1 Piaster / 25 Heller.
Sent from Jaffa to Baltimore MD on August 4, 1903.
Transfer at Paris, France on August 10, 1903.
Baltimore, MD Postmark with large numeral 7 and 1903. (What is the large 7 for?)

Arrival Postmarks:

N.Y.B. WASH R.P.O. - 20 AUG 1903 - IR77 - BALTO, MD
Baltimore, MD REC'D - AUG 20 - 7 AM - 1903
Baltimore, MD - AUG 20 - 8:30 AM - 1903
Princeton, NJ - AUG 20 - 6 PM - 1903 - REC'D

Thanks very much.

I suspect it's an inspector's mark. I'll be anxious to hear from the experts!

Baltimore would have had city delivery from c1865, so the address should have had a street number on it. I suspect this letter went directly to the nixie clerks (clerk #7) in Baltimore to determine a street number, which instead, they determined the need for forwarding to Princeton, NJ.

Thanks John, that certainly sounds like a plausible scenario. So it would have been sorted on the train where it was first canceled with the RPO postmark?

I'm amazed at the speed of the delivery, they really worked to get that cover to the destination in time and all on the same day.

Today airmail from Israel to the USA takes approximately 10 - 16 days. This cover took 16 days to arrive in 1903.

Just a couple of minor corrections:

The reference should be -- N.Y & WASH. R.P.O. (Ampersand, not "B").
Also correct the reference -- "IR77" should be "TR77" as in "Train No. 77".

Thanks everyone for their help with my cover, it has been very educational.