The 601 series came out in the '58 model year as a replacement/upgrade to the 600 series. In reality not much was changed. Slight boost in the hp, power steering became more common, but still optional. Live pto on the 661 model only. Diesel engine became available. From the factory, the 601 series was painted almost all red with gray grille, wheels, fenders. The gas engine is a 134 cu in and the diesel is 144 cu in. Most common models in the 601 series are:
631 - 4 speed trans and pto. 3 pt hitch optional, no proofmeter.
641 - 4 speed trans, pto, 3pt hitch, proofmeter
651 - 5 speed trans, pto, 3pt hitch, proofmeter
All of the above do NOT have live pto
661 - 5 speed trans, live pto, 3pt hitch, proofmeter
671 - Select-o-Speed trans, single speed pto (live or independent...however you like to define those terms)
681 - same as 671 except with a 2 speed live pto.
All of the above came in either gas or diesel. All are utility models with adjustable width wide fronts.
Below is a link that may give you more info.