This has become a running joke with myself, but I was going through my albums last night, and I pulled out this cover. It just happens that whenever I pull out a nice cover these days, it happens to be from .. wait for it .. Mobile.
This is a printed circular attempting to pay the 6c circular rate via Prussian Closed Mail, to Augsburg, Bavaria (Germany). I can't think off-hand of any other stamped cover I've ever seen paying or attempting to pay this rate. At any rate, its really rare.
The cover did not go via PCM, however, instead, it went via British Open Mail, but during the 3 months period - Jan-March 1857 prior to the US French Treaty. Such covers bear "tray" markings, like the GB 2FPK one on this. Regular letters sent during this period via British steamers are common enough, but this is printed matter. Circulars sent during the 3 months period are very very tough items to find. At one point I saw a census stating around a dozen or so were known. Probably more have been found since, but these are really tough to find.
So this cover was attempting to pay a rate for one route which would be rare stamped, but instead was sent via another only slightly less rare method.
Postmarked Mobile red CDS Jan 27 (1857).

edit: I should add that this circular was carried via British steamer. American packet circulars during the 3 mos period are right there with 6c PCM circulars - super rare.