Nice work - saw a kid pushing one of these through town earlier as it happens; recognised the questionable decor but couldn't place the model / where I'd recognised it from
I'll stick my oar in at late '80s / early '90s. I have a '91 German catalogue and all the European "cooking" 18-23 road models have Unicrown forks with lugged frames. I also have a '90 lightweight catalogue that shows the cheapest model (Vitesse) with unicrown forks combined with a 531 lugged frame, while the higher-end models still have crowned forks. The '85 road bike catalogue shows crowned forks on even the cheapest models, while my '87 Routier (a cheap model) also has crowned forks. Unfortunately I don't have any catalogues for the '86-89 period.
I'd expect the lower-spec models to have got the unicrown forks first so I'd reckon they came in in probably '89-90. Granted it seems that changes / advances in spec were staggered across the range (obviously with the rot setting in at the bottom and the improvements starting at the top) but by the time the lower-end lightweights were getting unicrown (in '90) I'd expect all of the lower-spec models beneath to already have them.