I just did some checking and the Deck belt and PTO belt for the M60 KW and KH 60" Tank also fit the Tank S series, the S 6031 and the S6031LP. The drive belts on the S series are different. So I'm guessing that all the 60" decks are using the same deck and PTO belts. If you know or have all the info on the mower I'd just call the nearest dealer.
You said you bought the mower from Lesco. Is your mower green in color, the Lesco Z-Two by chance?