The whole issue of 'phases' can be a quagmire; it is an invention of the hobby, not the manufacturers. I'm pretty sure the concept can be traced to the periodical "Extra 2200 South", and is used mainly to distinguish differences over the production life of a particular model. Differences that can be identified by so-called 'spotting features'.
That said, "Extra 2200 South" covered the C-420's in its Oct-Nov-Dec 1977 issue (it was a quarterly). Their detailed production roster was broken down into two 'phases', I and II. The main diference was in how the long hood handrails were mounted; for phase I, the stanchions were mounted ON the walkway, with readily seen holes (one under each stanchion) for access to the mounting bolts. On phase II, the stanchions were bolted to the sides of the side sills.
Within phase I, Extra 2200 South identified a minor change to the carbody air intake (the screen just behind the cab) within phase I production. Somewhere along the line, someone in the hobby decided that this change was enough to warrant a sub-phase; hence phase Ia and Ib.
In April 1987, Railroad Model Craftsman magazine ran an extensive review of the then-new HO MRC Alco C-420 (the author's name escapes me- the issue is buried here somewhere). Using the same detail differences, this reviewer decided on three distinct phases, I, II, and III (corresponding to the previous Ia, Ib, and II).
I referred to the 'RMC' phases.
Fuel tank capacity was not a phase spotting feature; it was a customer option, so any size could be seen under any phase. (Capacities were 2000, 2400, 2600, 2700, and 3100 gallons; for steam generator equipped units, the tanks were split between fuel and water. Capacities used under C-420's were 1000 Fuel/ 1900 Water, and 1600 Fuel/ 1500 Water gallons).
For even more confusion, L&HR 21 and 22 were built with a never-repeated pre-production engine air inlet (that's the one at the rear, just ahead of the radiators). It was a single opening; the production version was a pair of openings, mounted slightly higher. When the 21 went back to Alco for its post-wreck rebuild, it came back with not just the larger fuel tank, but the later version of the intake.
21 and 22 were both phase I (later known as phase Ia, or just phase I by RMC); the remainder were phase II (Extra 2200 South; phase III per RMC).
Now that everyone is thoroughly confused...