Is this a 2wd tractor? The summary sheets show that it was built that way. But they have been wrong before. The codes I know for sure are:
657 - regular speed 2wd with outboard planetaries.
658 - high speed 2wd with outboard planetaries.
661 - regular speed 4wd with regular rear axles.
662 - high speed 4wd with regular rear axles.
665 - regular speed 4wd with outboard planetaries. (Extra Heavy Duty)


Following the codes above, I would assume that 666 would be high speed XHD, but maybe they didn't like that number and jumped to 667. The high speed option used the same ring and pinion, but had a faster 4th/6th gear pair.