There's been a few threads about them, 18DAI posted one that garnered some good info:
Melonite 4566
It seems to be so far that they were LE. I bought mine in 12/11 and lettered it. No verification that it went to an LE agency, or who it was made for. The letter had a real interesting mention of it being part of a group that had been in S&W inventory since 1999 until being sold to various S&W Distributors. Mine was shipped to Williams Shooters Supply in Quincy, IL.
Makes me wonder how many guns comprised that group held back in inventory, and whether or not they were kept for use as warranty replacement stock.