Here is a web site that gives detailed instructions on how to increase the capacity. I believe that the magazine tube has three indentions that restrict the number of shells. You can file the indentions smooth on the inside of the tube to allow additional space for more shells. DON'T try and bend the indentions out, because you could bend the exterior of the tube and not allow the proper function of the gun when ejecting shells. http://www.wisnersinc.com/additionalinfo/remington_1148.htm
By the serial number and markings on the barrel.
You can sell it to a bunch of gang bangers for like $400 and some crack rock.
Remington's website has a history section that you can look at.
The value of a 20 gauge Remington Sportsman 58 would actually depend on a number of things. A couple of those things would be the age and condition.
You can call Remington at 1-800-243-9700 and go thru the menu selection. One of the menu options is to request information on your Remington based off the serial number. You read off your serial number and the guy looks up in the database the year the gun was made and what the original barrel was for that gun (if gun had barrel options like shotguns do).
Stamped on barrel, Barrel and reciever one piece.
CHeck the receiver and barrel
Please check the link for the Manufacture Date codes details on the Remington Society of America.
They have date code stamps on the barrel which can be looked up on the manufacture date link on the Remington Society of America. --- Update: It's important to note that the barrel on the shotgun MAY not be the barrel that was originally on the shotgun at purchase. Check the serial number with Remington.
Should be on the action, Sportsman could be the 48 or 58 semi auto. Sportsman models have little parts left so try not to break any.
The barrel date code is the primary method of dating a Remington. Of course, if the barrel is not the original barrel for the gun that complicates things. The Remington factory could probably date it by serial number. sales@countrygunsmith.net
There is no model number 3006 or 243 in the Woodmaster line. Check Remington's webiste under the Sportsman's Library.