The "TA" is a torque amplifier, and it allows you to downshift without changing gears; it gets you more torque if you need it as you're moving. There should be a 3 foot (or so) handle with a release lever on the left side of your tractor that connects to your clutch control through springs and levers. It also connects to a shaft in the chassis located below the gas tank. I'm going off the top of my head of how my Super M-TA is set up.
Your serial number tells the year; you can pick up a data book that will list what year your tractor serial number was issued.