Here’s another view...
The sheriff is correct, there is no way to determine when a magazine was purchased (and in many cases when it was made). The legislature assumed that since this is true LE would simply confiscate forcing the owner to 1) forfeit them or 2) show up in court and press his case THEN go to court again to get his property back if it wasn’t destroyed/sold/stolen. In short the legislature was depending on corrupt LE to carry out the unwritten plan.
"One, he says, he had before July 1, when the law banning the possession, sale or transfer of the large-capacity magazines went into effect."
If the new law bans "possession" of standard capacity magazines, why would the date of manufacture matter?
As an aside, even the draconian new gun law here in MD. which bans the "sale, transfer or receipt" of mags with caps greater than 10 does not disallow possession of them.
Yet.