[Arran]

Mike;
 They did the same in some post war Addison radios, field coil in the negative leg, including the catalin ones, and those were AC/DC radios. The theory I heard was that it would reduce the current flowing through the field coil, plus they could use the field as part of the voltage divider. I also have sets where the speaker filed is connected directly to the chassis on one end, meaning that they were filtering nothing, (choke did the filtering) only connected that way to energize the coil. 
Regards
Arran