You have to be careful. Here is an oddball roundback. This one has a GROUNDED brush installed at 9 o'clock, an isolated brush at 12 o'clock, and a blank for an isolated brush at 3 o'clock. I assume this is an aftermarket piece. The danger is if you were to have all three installed, and they try to use it "isolated." (70/ later)
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This is the "pinnacle" of a junk rebuilder who modified a grounded brush roundback to take a second isolated brush. This one also has a grounded brush installed. All they have done here, is drilled a hole in an older case The normal isolated brush is at 3 o'clock, the installed grounded brush is at 6 o'clock, and the unused drilled hole/ modified isolated brush position is at 12 o'clock
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This is a 70/ later isolated field unit before squarebacks came out
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