it's a reslo(sound) ribbon,
i'm surprised you don't know it, given that it's definitely an amazing mic that everyone should own. As used by the beatles!!
All of the above comes from ebay seller descriptions

they sell for daft money on there, i guess cos of the look or the history. every review i've read says if you find one it'll be broken! and if it's not broken it'll be crap anyways
never used one myself though.
pic courtesy of M.joly.