Ia- three latched engine room doors, older fuel tank profile, older radiator grills, sets of three louvers on both battery boxes, older Flexicoil trucks. [6/64-8/64]

Ib1- seven latched engine doors [9-10/64]
Ib2- two louvers on the fireman's side battery box door [11-12/64]

IIa- wire-rake radiator grills [1/65-2/65]

IIb1- inertial intakes change to corrugated type, dustbin hatch changes to the raised type [2/65]
IIb2- two louvers on both battery box doors [2/65-6/65]

IIc- fuel tank profile changes to the slope-top style, older handrail stanchion arrangement [5/65-8/65]
IId1- "double" stanchions by cab and rear stepwell [8/65]
IId2- front lifting pads moved forward under the battery boxes [8/65]
IIe- stanchion nearest the cab directly over the fuel cutoff switch, and others moved forward same distance [8/65-11/65]

III- 40-series turbo stack, air reservoirs moved outward, associated piping eliminated, front lifting pad moved back under the front side of the cab [9/65-1/66]

Many late Phase IIc and all Phase III SD35s had the newer Flexicoil truck design (FC-2L), but the older design (FC-1C) was still available as an option so the trucks are not a phase marker.