No
All.
If you have a 500, and it is a 12 gauge, it is a 500A. The "A" designates it as a 12ga. A 500B is a 16ga, a C is a 20ga, and an E is a .410.
It means you have a Mossberg Model 500a shotgun with the serial number L109563.
Yes.
It can be, if you are using a slug
175-350 USD
Yes, it is. The 600AT was simply sold under the New Haven name instead of the Mossberg brand. It's a Mossberg 500A (12 gauge) intended for targets and hunting (long barrel and great wood). All Mossberg 500 parts fit it.
You can determine the age of a Mossberg 500A shotgun by checking the serial number. Mossberg's serial numbers typically include a letter prefix followed by a series of numbers, with the letter indicating the production year. You can contact Mossberg directly or visit their website for more detailed information on decoding the serial number to determine the firearm's age.
50-250 or so
The Mossberg and Sons Model 146B was manufactured in 1954-1958.
I have sold a dozen of them in good condition from $125.00 to $175.00.