I flipped through most of your images and can give you some "general" hints about the relative pluses and minuses. When looking at Stampless covers, these are some good "rules of thumb:"
- Postmarks that are not the standard round, city, datestamp are better. In your case, you have a couple of "straightline" datestamps, a Richmond with an attached "5" rate marking, and some round datestamps that are not the "same" as the others, such as the one from Mississippi. This is a good start for the "better" pile.
- Earlier is better. Stamps were not required until the late 1850s. That leaves a lot of time with lower rates (e.g. 5 and 10c) Early 19th century, or even 18th century are better. I'd look within the letters for an indication of the date.
- Smaller, more unusual towns are better. Lots of mail from New York, Boston, Baltimore, Philadelphia, etc. Less mail from Texas, Florida, Mississippi, territories, etc.
- Odd rates are better. Best thing to find are the War rates and the Restored rates. By war here, we're talking the 50% surcharge to postage rates put in place in Feb. 1815. In March 1816, the rates were restored to the prior level, but in April, they were changed. That 1-month rate period is a good find. (FYI - rates can also help date the covers - see
http://users.westelcom.com/gestus/PHrates.htm for a simple list of the early rates.
Now, looking back at your scans with some of this in mind, the ones I'd take a second look at would be:
#2 - Washington, DC - what is the date? 50c rate is less common
#3 is a "Free Frank" but I don't recognize the name of the person signing it
#9 - Baltimore straightline. Date? There notation at the top left "4/4" is interesting. Would need to figure out what it designates. potentially it is the number of the correspondence. Potentially, it may be some British rate marking. I don't know.
#10 - Wilmington stratightine. Date?
#11 - "Free" is interesting. Can't read the postmark
#18 - Nice red Richmond attached rate stamp (by the way, all of these were sent "postage collect." Unless they say "Paid" or "Free," the recipient would have had to pay the postage.
#20 - Hernando, Miss. Unusual datestamp. (Date of letter?)May be of interest to someone who collects Mississippi.
#24 - another Baltimore straightline.
People have already commented on the Bloods adhesive on the Nesbitt Postal Stationary piece. That's nice, but remember that it's hard to get full catalog value for anything.
Lastly, you can try to request the American Stampless Cover Catalog through your library. It might give you a better sense of some of the markings.
Good luck and have fun with your research. Sorry that this is about the limit of my knowledge in this area and I can't help you more.
Chip