super mag receiver

[novaz]

someone local to me is selling an 870 that shoots 2 3/4,3.0/ and apparently 3 1/2 shells but only with a super mag receiver according to the script on the barrel a friend who lives aways away is interested in this gun but I don't know how to identify the receiver required for the 3 1/2 shells
any help would be welcome.
Thanks

[Synchronizor]

If it's a complete gun, look at the bolt in the ejection port. A SuperMag 870 will have a sliding port cover on the rear of the bolt to keep the longer ejection port closed off when the gun is in battery. Standard 870s don't have this sliding cover, the bolt by itself is enough to close off the ejection port.

Standard 12ga 870:


SuperMag 870:

[novaz]

Thank you