Ia- three latched engine room doors, older fuel tank profile, older radiator grills, sets of three louvers on both battery boxes, flat under the front headlight (roads that used rotating lights had a bulge under the numberboard assembly), older Flexicoil trucks. [6/64-10/64]

Ib- seven latched engine doors [9-10/64]

Ic1- bulge under the front headlight becomes standard [10/64?]
Ic2- 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.