They're just the designations for the upgraded/newer model of 2001.What does SRN/SRNB/DC/AC/130 even stand for anyway?
With the 2001*130 meaing it has reached 130 Decibles. **Edit**
more or less
starting with the origianal 2001 I think it was rated at 127 dB(C) and as time went on they (Federal) kept on improving the model.
The 2001 AC means it runs on AC power supplied from a land line.
The 2001 DC is that is runs on a battery back for main power / with ac power being used only for charging the batteries.
Edit: The SRN*B was an inprovement replacing the rotator belt device with a new beltless rotator device, & some other inefficentcies.