Not to split hairs, but 621 was actually a DL-107. Alco, like most teakettle builders who transitioned to diesels, didn't think in terms of standard designs like EMD did, at least initially. A change in sheet metal, alone, would have been enough to trigger a change in specification number, in those days. A formal set of model numbers (such as the PA, FA and RS series) would not be developed for several years.
In addition to that, there were a number of differences (most of them subtle and internal) among the early Alco passenger units, that distinguished them from each other, at least from the standpoint of those who had to build and maintain them. This is one of the reasons units like these became orphans, and were among the first to be scrapped, traded-in or (as in 621's case) re-powered. There's a good summary of the history here (http://en.wikipedia.org/wiki/DL-109
), in addition to which, RMC ran a set of brief articles with drawings back in the late-'60s, just after the last of these (IIRC these were RI 621 NH 716) and had been retired and scrapped.