Armi Marocchi offered a model SM 28 L sidelock sxs shotgun from 1983 -1986 according to the the Gun Digest Book of Modern Gun Values 16th edition page 589. Links to photos include:
http://guns-catalog.waffe.ru/64/599.htm
http://guns-catalog.waffe.ru/64/600.htm
I used the Google search with Russian to English translation.
I'm not saying your gun is a SM 28, just passing on these pictures for comparison. HTH ? Fred.