Model 183K-D Bolt Action Repeater, change to barrel and front sight. Made 1968-1986.
I do not have the info to narrow that down beyond the model years- HOWEVER- check Havlinesales.com. A polite email to the folks there MAY get you a date. The folks there are Victor and his wife, Cheryl Havlin. Besides selling parts for Mossbergs, they run the Mossberg Collectors Assoc, and are very nice folks.
The choke SHOULD be a variable C-Lect-Choke, and SHOULD turn. Would suggest a daylong soak in a good penetrant oil, such as KROIL.
Values on the bolt action shotguns are low- typically $80-$100. The .410s go for a bit more. Here is a nice one from Gunbroker for $141. http://www.gunbroker.com/Auction/ViewItem.aspx?Item=323366577