I don't live in CT, but I think they had a magazine ban implemented in 2013
Since 2013, Connecticut law prohibits people from selling or purchasing high-capacity ammunition magazines that hold more than 10 rounds of ammunition. Existing owners before the law passed were allowed to keep the magazines.
A person who lawfully possesses a large capacity magazine prior to January 1, 2014, must apply by that date to the Connecticut Department of Emergency Services and Public Protection (DESPP) to declare possession of such magazine
So if you had a mag before then and "declared"as jakeb stated, then you could keep it and use it (with some restrictions).
If he has any standard mags that he 'declared' then he could then buy/use a gun that could take advantage of those mags for CO.
Similar in MA, but the ban is the actual 94 ban, so if people have mags from before then, then they are using them for CO.