I believe this is correct. My understanding of Mass law is that ONLY factory configured SBS and AOW's are legal. And no alterations are allowed from that factory configuration. So, Mossberg AOW, ok if unaltered. Serbu AOW, NOT ok as it was not manufactured by the factory (moss/ mav/remgtn)..
Example, I have a factory Ithaca stakeout (reg AOW) with 14" bbl. But, I can not alter it in any way (send to Serbu to super shorty it). Even though federally, I could, legally, amend the form 4 to reflect a shorter BBL. Mass would consider this a sawed off shotgun.thus, verboten.