Hi, thanks for the comment on my build.
Nice bike, i did see it one DBA.DK but the dude newer returned my message.
i might know the original owner of your Plexus, i think it belonged to Lars working at Finn C (A bike part distro.) in Westend at Vesterbro.
i'm not really sure on the Plexus history there are from my knowledge several types of plexus' one made i Japan (i think only whit 1 1/8 head tubes) and a European produced "copy" made by FRM i France (both 1" and 1" 1/8)
yours looks like the France made version (I'm really not sure if thy were build for Ritchey or just repainted later to look like one)
there is to things that make me think so: the saddle bolt on the front of the seat tube and the bolt on thing behind the cranks
I hope that we to gather can track down some more info on the Plexus'