The following phases are based on what I have observed in exterior detail differences.
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- Phases -

Phase 1a - 1966.07 - 1966.12
-Walkway sideframe profile: Notched at ends
-Hood door near handbrake: Full-height
-End plate: Version 1 (Small tabs at bottom steps)
-Steps: Short
-End handrail stanchions: Flush, overlapping end plate
-Side handrail stanchions: Angular bottom supports
-Door hinges: 3
-Rear sand filler: Angled housing

Phase 1b - 1967.03 - 1968.03
-Walkway sideframe profile: Straight
-Hood door near handbrake: Half-height

Phase 2a - 1968.04 - 1970.06
-End plate: Version 2 (Angled sides at bottom steps)
-Steps: Tall
-End handrail stanchions: Angular bottom supports

Phase 2b1 - 1969.12 - 1971.03
-Side handrail stanchions: Straight bottom supports, ridges overlapping sideframe

Phase 2b2 - 1970.05 - 1970.06
-End handrail stanchions: Straight bottom supports, ridges overlapping end plate

Phase 2c - 1971.04 - 1971.12
-Door hinges: 2
-Hood door near handbrake: Full-height

Phase 3a - 1971.11
-End plate: Version 3 (Straight edges, no extension at bottom steps)

Phase 3b - 1971.08 - 1974.01
-Rear sand filler: Straight pipe

- Other details -

-Not all Phase 2a and later units had taller steps.
-Trucks (Type A or Flexicoil), side handrails (hood- or walkway-mounted) and fuel tank (600-gallon straight or 1100-gallon curved) varied across all phases.
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Michael Eby 2009