Your first question was answered in the previous post.
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Second is the triangle with the 5 over 6 inside a register marking?
This is not a registered marking. It is probably a censor marking. I can't say from where, but probably Russia.
The same for the Boxed "23" on the reverse. Likely censorship.
The censor label is probably Great Britain, but could be from anywhere that there was a British Colony censorship station along the route. I don't think mail was routed via Bermuda, etc., during World War One, but that is a possibility. One would have to know for certain where Examiner #4057 was stationed at the time of the letter, which is beyond my resources..
The violet boxed REGISTERED #248 is the US registration mark and letter number added at the office of mailing, as is the Double Circle mark on the backflap at top left: "WILLIS MASS /REGISTERED." This is in accordance with US registration regulations that no date stamp is to appear on the front of a registered letter, but must be on the reverse and along the backflap. This regulation was to force the postal clerk to verify the letter was properly sealed when received.
The two New York oval markings on the back: "New YORK / REC'D (9) FOR'GN" were applied at the Foreign section of the New York Registry Division, and were the result of a clerk again verifying that the letter was properly sealed.
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Lastly the red pencil markings including the #118 is that the censor? You can see the 118 also shows at the top of the first page of the letter.
I think it is likely that the "118" is a letter number, indicating this is the 118th letter sent in a series. The recipient probably added the #118 to the envelope after receipt and reading the letter, to make it easier to keep in sequence.
The red pencil with "29 / x11/17" is probably the date received, and was added by the recipient.
The red pencil "85 ???" is again probably a notation by the recipient. Perhaps a note as to cash received with the letter?