I like the thoughtful way in which you mounted this "ex-cabinet" LS Chris! Unless I'm seriously mistaken, the consensus seems to be that it's actually a permanenat magnet type-in which case what happens when you wave a screw driver at it?
I've always known that early speakers often had their own power source. I only came across the designation "Mains Energised Loudspeaker" though, not "Field" but [of course] the electro-magnetic creates one! It's certainly a fine example either way but given the vintage and "ad hock" production regime at the time, you may never locate it's original home. Based on past experience though, just me saying that, in the way of things, may result in a positive ID from a member
Dave W