First off, welcome aboard!
To answer your question, the different voltage options were available solely to fit the available power at the site. For instance, a factory is more likely to have 240V power easily available throughout the facility, so a Model L-240 would be chosen. Or, as a street-clearing siren, is 120V power is Probably more easily available, so an L-120 is best fit.
As far as sound output, they are the same; voltages don’t really affect the sound output because 120 and 240 volt motors spin at the same speeds, just at different voltages.
Also, in the earlier years, Federal Electric would be able to offer the Type A siren anywhere from 6-250V!
Very proud owner of 2 A’s, 2 D’s, J, 76A, Interstate Machines F, & a Sparton 35
"Coughs an sneezles, spread diseaseles!"
21, & lovin’ sirens till the day I perish