Looks like there were 2 different engines for the 600, is that correct? both look to be 440's but different manufacturers. Is one better than the other?

I have zero experience with John Deere sleds but have the opportunity to get a 600 for a good deal. I am trying to figure out the year to determine how hard it will be to find seals, gaskets, and the parts ill need to get it running. I was told the serial number is 00515m. Could anyone tell me what ...