There were at least nine Federal Signal 2001 variations over the years. The original prototype design had a completely circular intake box, however I'm not sure if any of these were ever produced. Up next came the "Roundback" 2001-DC. (I use 2001-DC to identify the 2001 sirens that, in fact, say 2001:DC on the rotator. Just letting everyone know before they get all riled up about it.) Next up was the regular square-backed 2001-DC. The previous two 2001-DC's both had conical motor housings with large Federal FS decals on them (most of which are weathered off by now). They also had weather screens. In between this version and the next, there may have been a 2001-DC produced with the conical housing that did not have a screen, such as the ones located in Goshen, IN. This was followed by a third "2001-DC," that resembled a 2001-SRN in that it contained the same motor housing, but the intake ports were narrower and it still listed "2001:DC" on the rotator. This version had a small Federal FS decal. Next up was the 2001-SRN we all know and love, which also has a small Federal FS decal on the motor housing. This was followed by the first generation 2001-SRNB. These bared all resemblance to the 2001-SRN except that their rotator was tall and cube-shaped. The second generation 2001-SRNB featured a larger Federal FS decal on the motor housing, and I'm not really sure what else changed. Both SRNB's had decals stating "2001-SRNB" on the rotator. The 2001-130 followed the 2001-SRNB. The 2001-130 can be identified both by the tall, rectangular intake box and the decal that states its name on the rotator. These also have large Federal FS decals. The Equinox then followed the 130, although not everyone considers it a part of the series. I do, as there are complete systems of them, so it wouldn't be one custom order. The Equinox looks identical to the 2001-130, except that it reads "2001-Equinox" on the rotator. The Equinox is the 8-port version of the 2001-130. Hope this all helps.
And
@E57, this should be of assistance in finding Sentry model numbers:
http://www.sentrysiren.com/warning-sirens-products/
The only siren I can think of that isn't mentioned is the 5V, which is no longer produced. It's basically the 5HP version of the 3V8, only the skirt was larger and it came with a 16 port option.