Hmmm... This string keeps coming up with more information, but I'm getting more confused!
(1) The Alco C424 (with the GE GT581 main generator: the same generator used on 12-cylinder 244 and 251 engined locomotives with power ranging from 1600 to 2000, so maybe close to its limits on the 2400 hp, 16-cylinder, C424) and C425 (with the bigger GE GT598 main generator also used on GE's own U25B) are very similar in appearance: slight difference in the sheet metal around the radiator compartment the only "spotting mark" I know. SLRC 181 (the unit in the second of the two pictures PEIR links to) has what LOOKS to me like a C425 radiator compartment. This is SOME evidence that it was built as a C425, though I suppose it is possible that it got a new and larger radiator at some rebuilding.
(2) 181 also has an extra air vent in the side of the long hood just aft of the cab, a vent NOT shown in PEIR's other linked photo, of sister unit 381. (D&M, the predecessor of LSRC, numbered its diesel locomotives by date of purchase: these two units, then, would have arrived on the property two months apart, in January and March of 1981: dates consistent with their being "graduates" of GE's rebuilding program at the former Erie Railroad shops in Hornell, NY.) I suspect this vent may have been added after the initial rebuilding: perhaps at the time a new (AC) generator was installed.
(3) I don't know of any application of the GE GT598 main generator to a locomotive less powerful than a U25B or C425. In particular, if some of the units that came out of the Hornell rebuild program started life as C425, I don't know whether they kept their original generators or got GT581 generators.
(4) As to the bulge: thanks, Bright Star, for the information that it was for a new crankcase exhauster arrangement. I think other rebuilds of Alco 251engined power at about the same time also modified the as-built arrangement-- in particular, I recall reading about a CN rebuild of an RS-18 that had a change of this sort.
Re: GEU28B and AC main generators. The two AC-equipped U28B test-demonstrator units built in May 1966 were the first two (301,302) of GE's four unit U28B/U30B/U36B test/generator set 301-303. GE apparently originally intended to build a four-unit set with two being alternator and two GT598 equipped, but by early 1966 railroad demand for high-horsepower units with DC generators had collapsed. (Some years ago a former GE employee said, on the Railroad.net GE or New York Central forum, that GE went several months without getting any new orders at that time: they had designed the GTA-9 alternator, and had supplied it to Alco for use in C630 units, but had decided to offer the U28B and U28C with DC main generators,and this turned out to be a mistake.) So in June they built two new units with alternators-- 302 and 303-- and sold the DC pair from their original set to the New York Central as NYC 2822 and NYC 2823 (the two NYC U28B in the new carboy,and the only U28B owned by New York Central proper, 2800-2821 having been bought by the P&LE).
As to later U28B being built with traction alternators: the old "DieselSpotter's Guide" had a somewhat vaguely worded statement on this, which I think led many people to believe that all U28B built after May had alternators. A few did, but most had DC generators: this was discussed several years ago at Railroad.net and "Loco-Notes," and someone copied out data from an internal GE document (the "Moser List") specifying which orders got which in a post to "Loco-Notes". I think L&N's U28B had alternators, but all of Rock Island's (including the late ones in the new carboy) had DC generators; I don't remember any others.
As to GE U23B units, a slim majority had GT 581 DC generators, but in the latter half of the model's production history the AC option was more popular. There was a detailed list of which got which in "Extra 2200 South" some years ago; I can find my copy if anyone is interested in details.
Alas, I know of no Kirkland- or Steinbrenner-style book on GE locomotives. (Greg McDonell's "U-Boats" (Boston Mills press, 1994) covers only the (domestic) U-series, and is more of a picture-album, with fairly thin text.) Kirkland was supposedly working on a GE volume when he died; I have been told that his materials are in the hands of someone who intends to bring out a book... some time or other.