Most of the elements of the frame here seem consistent with those of the Dutch marque JOCO. The cylindrical cable stops, Nervex/Ekla mix, non-matching dropouts/tips, the vertical head tube rivet holes (would be interesting to know the distance between them) and the serial number format/location/orientation seem to match.

I don't know enough about the serial number system JOCO used but there seems to be at least one person on oudefiets.nl who claims to know.

With all that said, the seat tube outside diameter here (if correct/accurate) is a little confusing...


Here are some pics of a Ronde van Nederland (different dropout and fork crown):










Another, unknown model (different lugs, but matching fork, drops)