Bump! I just got the pedal. Unscrewed it, attached the batteries and eveything seems to be working. Still haven’t got a clue when this pedal was made or what it’s worth, but I tried to take a couple of photos.

What immediately strikes my untrained eye is the absence of internal trimpots, and that the cirquit board looks a lot, hmm, “tidier” than the ones I’ve seen on eBay, but I’m not sure what that means.

Help, anyone?

Edit 1: This one looks pretty similar to mine, doesn’t it?
http://www.ebay.co.uk/itm/VINTAGE-ELECTRO-HARMONIX-ELECTRIC-MISTRESS-FLANGER-FILTER-MATRIX-EFFECTS-PEDAL-/190779464261?pt=Guitar_Accessories&hash=item2c6b579a45

Edit 2: I took some new pictures with an actual camera instead. I also found the following printed inside:

Cirquit board: EH 5100D
Filter switch: 51-505 STACKPOLE UND. LAB. INC. LIST.
Color pot: P0130 1378038
Range pot: P0150 1378038
Rate pot: P0200 1378044