You will have to contact Mossberg
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.
The value of a Mossberg Model 500e 410 gauge shotgun overages between $200 and 300. The actual value of the shotgun is largely determined by its overall condition.
50-160 USD
$300-$450 depending on the condition. Based on a Mossberg 500 and manuf in 1973
255.00
$250
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.
I'm sure it has sentimental value to you, but I cannot see it to assess its value.
75-250 USD
about $100
75-200 USD