How long before the second purchase did you buy the first one??? That's really unique that their serial numbers are one right after the other!!!! Seems like if it was a significant amount of time between purchasing the two mowers either (1)that dealer isn't selling very many of them for you to get "two manufactured in a row" or (2)no one who purchased one of these from that dealer received that specific mower(the second one) when they bought one.
What are the differences? Well there are a few like the Exmark is 170lbs, the TM 167LBs. The Turf-Master has a 13gauge steel deck, the Exmark has a Stamped steel deck with some welded reinforcement, the Turf-Master is 2 point height of cut adjustment the Exmark is three point and the Turf-master is a single bail control system and I think the Exmarks are two bail. I mean there both made by Toro so in reality there basically the same mower with one having a few more features than the other, the engines are the same ,BTS is basically the same, Exmark say approximately 18500 and toro says 18000. The Toro comes with skid plates and I'm not sure about the Exmark. The Toro is also a variable speed transmission and I'm not sure about the Exmark.
