Hi Don.
Here's some info on the MT 31. Mann MT31
Please excuse the obvious sales pitch theme of that site but it's to only information I can find.
I picked mine up reasonably cheap from a dealer friend a couple of years ago. (They retail here in Aus for about $1500). I've found it to give me good results, especially on female vocals and accoustic guitar. But then, as I have nothing to compare it to, it could be a piece of crap too