NYS still has its version of the assault weapons ban on the books (See Article 265 of the Penal Law). This law includes large capacity ammunition feeding devices (magazines holding more than 10 rounds) made after the effective date of the federal AWB. A magazine marked "For Law Enforcement Use Only" may be illegal under NYS law, if it was made after the federal assault weapons ban took effect (Sept. 1994).
I bet the numbers on you magazine are date stamps, but I don't know.
Those magazines are probably legal in most states, but probably not NY.