i used to know a whole lot of awesome sites on this subject...
but that was a few years ago, and i forgot them all, except one:
www.hkpro.com
you'll find everything about heckler&koch guns there (check the "world of HK" link!) :-)
directlink to mp5sd from bhd:
http://www.hkpro.com/mp5sd.htm
but there are plenty of really good pages out there, maybe you only picked bad ones ;-)