While my Batavus frame is still at the blaster/welder place, I found a Burco "opafiets"!!
It's probably an '50s "opafiets" (Grandpa's bicycle).
The brand is Burco, from Amsterdam.
I'm really stoked with this "path racer" fever, so I made some changes and now it's called "Opa Racer".. :mrgreen...