The UMC brass shell you note is relatively common. You will find it in a number of lengths and gages also taking either Bredan or Boxer primers. Other variations might be nickel plated vs plain brass, plus variations of the headstamp in letter spacing, placement of the "A" or letter font. Another not so common variation is that the shell might be fitted with a push-out anvil for ease of reloading. A reasonable collection of just this headstamped shell can number 20-30 different variations.
The "A" is for guns chambered for both paper and brass shells. If it was a 12 "B" gauge it would be for guns chambered for brass shells only.
My 2¢ on value to one of those in 2 1/2" length is about 3 to 6 US dollars for the common variation. Now the "B" variation shell is harder to find & one of those might be worth $15 or so. Just my opinion.
On the IAA home page under the REFERENCE link there are two items you should look at: one is the ammunition catalogs. You can download a lot of great FREE information there. Also there is an Introduction to shotshell collecting you might find of interest.
Another thing is to look under the IAA EVENTS link on the home page for an ammunition show that might be near you. You ought to find that shell at most any show.