I need help finding out what year my ST/10 is?

I have just got and fixed an old ST/10. The model # is 143.649012 and the Serial # is 3163E. The tractor is yellow and white and the engine says that it is "timken bearing equipped". The one feature that it has that I have only seen once is that it has a small foot rest that is at an angle and it does not have a flat base. It looks just like the the tractor that is labeled "something different" in the Suburban Pictures. If anybody can help me out it would be appreciated. Thanks

They were made on and off between 73 and 76. It was the last incarnation of the Custom series of tractors.