I've never understood the legal genesis of the "if it shipped with" theory. The law doesn't appear to include any such wording, so I can only assume that's someone's interpretation. I don't see how it could be correct - the MGL is very clear on this one.
EDIT: I see that wording in 501 CMR 7.09, I just don't understand where in the actual law it came from.
It's on the Large-Cap Roster here:
This roster has been compiled in accordance with M.G.L. c.140, §131¾. It contains weapons determined to have been originally manufactured for the civilian retail consumer market as large capacity weapons as defined by M.G.L. c. 140, § 121
So it was added by GCAB and approved wording per Secretary of Public Safety.