The Cub Cadet started at 501, the first 25 were test models and a few of those are still around. I know where SN 777 is.
At least 3 of those units were at one time in Massachusetts owned by the same person.
They did make 10 prototype units that were shipped out for testing and the results recorded. They were then shipped back to IH to be evaluated. Those tractors were then hand stamped with the serial numbers 401 thru 410. One of those tractors #409 was bought by an IH employee and used at his home. It has survived and has been accurately restored to its original form and is held in a private collection. It sports pieces without casting numbers and a k161 Kohler with a spec number designation of "EX". It was confirmed by Kohler in 2003 that the EX designation indicated experimental at the time the engine was built.