Quote:
What if the stamp fell off in transit resulting in the postage due charge?
A valid point to examine. The 1913 Postal Laws and Regulations, which would have been in effect in 1918 cover missing stamps in paragraph 2:


So clearly another stamp was on the envelope when it went through the canceling machine. Likely a 1 cent stamp, this being the 3 cent war rate period, 1917-1919.
However, I think in this case, the letter was double weight (note the irregularity to the wavy lines to the right of the stamp likely caused by a bulky letter), paid 3 and still due 3, and the absent stamp lost after delivery. Had the stamp been missing (torn off by the machine, for example), there should have been a notation by a nixie clerk of "postage ok, 1 rate" or something to that effect as part of the postage due rating process. The absence of such markings is why I conclude the stamp was a 1 cent stamp lost after delivery.