Answering an off-board question -
Yes, there is something missing from your cover - that would be some sort of rating. There is a ms 'pd', which indicates that it was pre-paid, but there should be a '5' or a stamp there as well to show how much.
Look at this cover for comparision -
Siegel sale 1155 lot 3156:
https://siegelauctions.com/lots.php...155-lot-3156Your cover shows the registry number '55'.
Your cover, at a single letter rate should have cost 5c to mail.
So its missing either a '5' or a 5c 1847 issue stamp to indicate pre-payment. That seems to imply it once had a 5c stamp on it.
From a valuation point of view, this is a serious flaw. A missing stamp is a bad thing on any cover. It is still interesting for study, however.
Dr James Milgram wrote a nice article on these in Chronicle 249, but its recent enough to where its behind a paywall, so I won't link to it. He counted 39 of these franked with stamps, and he alludes to the fact that there are even more stampless.
So this isn't a rare marking, but it is mildly scarce and obviously interesting, so there would be some small value in your cover. I don't think your cover needs expertising, as I don't think there is enough value in it. At this point, w/o the stamp, its a faulty stampless cover, with nominal value in my opinion.