You have to go back quite a ways to see the A and B shells. This from the 1880 Union Metallic Cartridge Co. catalog --

Attachment 57515

and 1885-86 --

Attachment 57516

mention A and B sizes.

By the 1887 UMC catalog there is no mention of A and B sizes --

Attachment 57517

So, are the Viet Nam era brass hulls worth anything to reloaders or are they trash?

They must have been used early years of Nam. All I ever saw were od green plastic shells in 1969.

The outside diameter of the 12A brass shell is the same as the standard 12 gauge paper shell. The 12B brass shell is smaller than the 12A. Guns chambered for the 12A will handle 'modern' paper and plastic shells. There's a little more to it, overall, but that is it in a nutshell.

parker brass

The Viet Nam brass shells were left over from WWII. When they got short in supply they went to plastic shells.

I had several 10ga. that were headstamped Parker Bros Meriden,Ct.They all sold for between $100 and $175 each.Regular brass 10 or 12 gauge shells without the stamp will sell for around $30.Any shells with major gun company stamps will bring a premium.

What was the significance of nickel plated shells? These two NPE shells are both 12A, but the brass one is 2 9/16" and marked West Meriden, while the nickel plated shell is 3", and the headstamp has the diamond surrounding the primer, and is marked Meriden Ct.
My understanding, whether correct or not, is these were all made by UMC, in Bridgeport.

Here is a UMC 10A for comparison --

Attachment 57658