whats up with this odd 2 brush generator?

[cas3]

bought this at the swap meet today for 5 bucks! it appears to be the later style 2 brush genny, but only has one terminal on the case, marked as "field", so how can it work with no armature post? green book shows a photo on page 456 listed as 81a 10000d, which shows to be 38-39, and perhaps 60hp. shown in my photo, oddball on top with one post, who knows in the middle, 2 brush with fan mount pulley and 2 connections, and on bottom a standard 40-48 2 brush with a tin pulley and no fan mount. how was it regulated? i am planning to build a 12 volt thing using the 56 and up case and armature on this front mount. thanks, skip darn, its hard to see in the photo, the top one has only one post, the other two have two