ESS Bookshelf Speaker ID?

I picked up a very small set of ESS speakers today on my travels.It has a 4 1/2" woofer and a 1 1/2"(or so) tweeter.The funny thing is that someone took off the round back cover on each of them.Does anyone know what model this might be,and if its just a wood cover,or is it a space for a passive radiator?
These are nifty little speaks,BTW.
Swell day all around.ESS speakers for $6,a pair of Grado headphones for $15,and a really nice pair of C-V D-4's for $20.
Thanks for any help I can get.
Jimmy

How about some pics???????????????

I do believe that there was a passive disc type thingy on the back. No speaker frame as such, just a disc of some sort of material and a surround attached directly to the cabinet. By the way, those tweeters are the same as the ones in my cbs reference monitors as seen in this thread: http://audiokarma.org/forums/showthread.php?t=72999

I'd agree there was prolly a PR there, but it's not obvious how it was mounted - there's no screw holes. The passives in my Tempests consist basically of a styrofoam disc (covered in foam for pretty), a surround, and an aluminum ring frame. You can find replacements at SimplySpeakers and on eBay, but perhaps not one that small. Seems it would be easy enough to buy some surrounds and make one, though. - Mark