Yup, "Messenger" is just a label they slapped on the RD2's. It doesn't really make any sense, they're not even labeled as such, it's just an RD2 with the messenger ring. The info is at
http://www.suginoltd.co.jp/singlespeeder_e.html, you'll see that the chainline ("C.L.") is 45 for all but the 75's.