The SC and the SW were built simultaneously at EMC, though EMC was pushing the SW which had a fabricated welded frame ("W" for welded and "C" for cast). The SC had a cast frame which EMC could not produce in its own shop, hence the attempt to sell the SW over the SC. Though there has been some confusion between the two models in the past, close-up photographs have revealed that the MoPac units were indeed SC's with cast frames (road numbers 9000-9003). When welded frames began in late 1936, many railroad people viewed them as a cheap imitation for a substantial cast frame. Consequently, engineers at La Grange decided to disguise their welded frames by applying a fake sheet metal section at each corner of the frame. These appeared on the SW1's, NW2's and TR2's until April 1949, when a cost cutting measure struck them from the package.By this time no one cared about whether frames were welded or cast. In 1938 MoPac subsidiary Union Terminal Railway (UT) got into the diesel act by purchasing an SW from EMC, which was outshopped as UT-5. In December the Saint Joseph Belt (SJB) picked up an SW (SJB-10).

In the roster section of the book, the following is shown for the SW's: UT-5 became SJB-5, then MP 6005, built 05/38 (c/n 718), retired 8/65, to PNC, to Pickens, renumbered PIRR-3. UT-10 is shown at becoming SJB-10, then MP 6006, built 12/38 (c/n 751) retired 9/63, scrapped. So by process of elimination, the locomotive shown MAY have been MP 6006, built as either SJB-10 or UT-10.