Yes, the 600 At is the same model as the 500 series, but with a different designation for sale in New Haven stores.
So yes, all Mossberg 500 after market or OEM parts will fit it.
No
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.
$175
The value of a Mossberg New Haven bolt-action 12 gauge shotgun actually depends on a couple things. Most important would be the condition of the shotgun.
I do not think Mossberg made a 220K. The New Haven 220K (single shot bolt action 22) cross references to the Mossberg 320K. Value is around $100-$125. Excellent first rifle for a new shooter.
Mossberg 500Not sure exactly how old, but it's really a Mossberg 500A. I think it's just the store brand from Sears Roebuck. I found one that was up for sale. Here's the info:Mossberg/New Haven 600ATCondition: Never fired. Price: $350.00 The New Haven 600AT series is basically the same as the Mossberg 500 series. New Haven is/was Mossberg's promotional line that was sold by other retailer (Montgomery Wards, Western Auto etc). Most all parts interchange IIRC. Mossberg barrels are rather inexpensive at gunshows and shops as there are so many around. This barrel comes with a factory installed "C-lect" Choke -- the cousin of the poly Chock of the 60's. Beautiful firearm and a fine shooter of the "old" family of firearms made by a Tradename Manufacturer for a company and their private label "New Haven". Kinda like Stevens making guns for SEARS. I think Montgomery Wards went out of business about 10 years ago, so one with New Haven one it you can probably safely assume is at least 10 years old.Please note $120 or so is a more realistic price for a used one. Check on gunbroker or auctionarms
You will have to contact Mossberg
Think you have a New haven 250K. If so, it is the same as the Mossberg 152K, made from 1950-1957.
Send it to Mossberg they can tell you anything you need to know about it and then some. You wont get alot out of calling them.
One can find a clip for a 22 caliber New Haven Mossberg rifle through local gun retailers. These pieces can also be found through gun classifieds and gun manufacturers.
The Mossberg plant moved from New Haven to North Haven about 1962 when the Interstate (91?) came through the old plant.
A history of Mossberg has been written and is available on Amazon