Harper, you ask the right question. To me this cover is a bit of an enigma.
If this cover entered the mails in San Francisco, or Sacramento (which it does not appear to have), then the DUE 10 would be perfectly proper, for an overweight letter requiring 20c postage to mail back east. The single rate was 10c. This was June 1860 or 1861.
The DUE 10 on the cover appears to match San Francisco's but Sacramento also had a similar stamp. Oregon House is a small mining town north of Sacramento.
The issue I have with this cover is that the town of Oregon House clearly did not have handstamps. Covers were almost always rated at the PO where they entered the mails. They might get re-rated at the destination PO if they are forwarded, advertised, or remailed. But to get a DUE 10 applied at some PO in-transit is unusual. Typically, even if a cover was overweight, but the origin PO missed that rating, I almost never see the destination PO 'fix' that. I put this under a glass when it arrived, and the DUE10 appears well tied to the two stamps, to me.
Now consider if the stamps did not originate. Then we have a stampless cover with a DUE 10, and we still have the
same dilemma with the Oregon House and a DUE 10. No difference in what I perceive to be the issue here.
The stamps are paying a proper single rate. Additionally, the stamps are pen cancelled, which seems to match the Oregon House pen cancel. It is tough to tell that for sure, as the cover I'm sure has been cleaned/restored as is the case with almost all of these kind. Often the stamps are not touched in that case, and you get some fading of the ink on the cover but not the stamps. Anyway, the Oregon House postmark and the pen cancel I would argue agrees well enough, so my current take is this was some sort of mistake by the Sacramento or SF PO. That seems unlikely and/or weird, I agree. Maybe Mt Carmel applied this, but I'm not sure why.
If this was mailed in June 1861, then it likely went via Steamer via Panama, since the Butterfield Overland Mail had collapsed due to the civil war, and it likely would have re-entered the US at New York. Now NYC shouldn't have had any reason to re-rate this, but maybe that happened for some reason.
Anyway, this cover has a 100+ year pedigree and I've seen very old photos of it. There were shenanigans going on with covers in the early 1900s of course, but in spite of the peculiarity, I don't yet see anything that tells me the cover is 'bad'. Just odd.
I welcome any input.
Another Oregon House - very similar style -
