This was hashed out many years ago in Boston courts.
A convicted felon out of prison got caught sending/getting/possessing many Air Gun moderators in the mail iirr. A gun ho prosecutor jailed him before court for violating parole. In court his case was dismissed and it was found AG moderators/silencers, LDC, etc, are not against the law even for a felon to have in his possession.
IIRR, judge told the felon, don't get caught with one of your AG silencers on a real gun...
So, imo, don't drive around with a ag silencer on your back seat not on a air gun, just to avoid any questions of what it is...
Even if it doesn't fit, wouldn't have it in the same lock box/glove box with your pistol, unless you enjoy explaining yourself to LEO/courts.
Sure you can do a search and find that case in Boston. It went into detail and was kind of interesting how the lib court made the decision...