Usa-1856 Portland Me - Stamp Missing

Here's a cover I was researching and mounting last night, which I found interesting. Part of a group of covers and letters I acquired a couple of weeks ago, sent to a Port Hope C.W. (Ontario) lawyer in 1856, I thought this cover from the US was stampless, in the 'pre-philatelic' sense of the word.




But what I realized is that there must have been a stamp which had fallen off in transit. The letter was still delivered. There is no postal rate written or stamped on, as would be with a stampless cover, and notice in the upper right corner, the partial cancel. It appears in the extreme corner, but also a small bit to the left. The resulting space fits exactly a US SC#68 10¢ stamp which would have been used to cover postage from the US to Canada (I have one in my collection, which I placed over the space).
But the notation of receipt written by the lawyers office is right over the space where the stamp should be. That tells me that either the stamp fell off on the way to Port Hope, or was removed, for whatever reason, after being delivered but before being filed. I have my doubts about that scenario.
Anyway, a bit of an interesting curiosity, I thought.

It is not uncommon for stamps to be separated from their covers in transit. In fact, there is a whole group of handstamps for that very reason. Here's an example:

A second look at the left side of the cover shows a postal marking which was used in the stampless period. I have a cover with a similar marking, shown below. Is there any date to this cover or its contents?

Stampman, there are several different types of that marking. Yours (if measuring 29x21mm) is reported used 1861-63, estimated to be from Detroit, where it made to crossing to Windsor. But the ink looks to be the same color as the Chicago cds. If so, it is more likely a marking applied there.

The barred circular killer at left is just another partial strike of the (partial) duplex cancel at right, if you were wondering.

thanks for the responses everyone. Yes the date of the is cover and the enclosed letter is 1863. I see I mistyped the date in my original post as 1856, not sure why.
You can see on the envelope, when held at an angle traces of the perforations, because the texture of the paper is slightly different. I think the stamp definitely fell off.

The missing stamp could have fallen off in the mail, or it could have been removed by someone at the company after it was delivered to them, and then it was docketed by some clerk or other person at the company after the stamp was removed. Postage stamps in the US were first introduced in 1847 and their use was optional with the alternative of stampless letters with appropriate postage amount markings until 1855 when their use became mandatory. This is several years after that requirement and there is no indication that there was any reason why this one did not require a stamp such as free franking or such. Also, there were stamp collectors actively collecting stamps from the very beginning - philately is not a recent phenonmenon. If the stamp was removed, I think a likely reason is there was a stamp collector at this company.

The 1855 requirement that senders prepay with stamps applied only to domestic mail. Foreign mail could still be sent postage due after that date. I'm not sure when prepayment was required on foreign mail, maybe UPU, but mail could be sent unpaid to Canada and the Provinces in the 1860s.

Thanks for the clarification, SPQR! I learned something new today.