I have the unit and manual BUT I don't know where the manual is. I
know its somewhere in the house.
I did find the manual for the M-450 which replaced the M-400. I know it added a serial PC connection and removed POGSAG and SuperPOGSAG and another one for pagers.
http://www.universal-radio.com/catalog/decoders/m450om15.pdf
I basically used it for DCS, PL, and pager decoding. If you use it for DCS or PL decoding, you'll need to tap the discriminator of a scanner for those and use the line level input for the pager decoding. I also know this thing does a lot more then I have ever used it for.
I'll look for the manual and scan it in if I find it.
EDIT: I cannot find the manual. I have limited knowledge of anything other than what I listed above, but will help if I can.