Nice find, @wschellen. Looks like a nice, well made frame.
I suppose it could be a Urago product but I haven’t come across one that has matching features. Also, as @juvela pointed out w/r to their frame number, I’d expect some sort of a stamp on the NDS dropout.
It's pretty much impossible to have a slam-dunk ID of this frame, so I’ll go out on a limb here and throw out one possibility: La Perle.
I have seen a few of their frames that share some similarities with the mystery frame here, especially that brake bridge that (to me) is somewhat unique. Here are some pics for comparison:
Brake bridge (the four comparison pics are from two different frames. There is a third one I've found but the bridge was almost completely obscured in the pics so no point in including it here):
Head lugs/cable guides:
Seat stay caps/seat cluster:
Fork crown:
Stay ends (by the way, what's French for "fish mouth"?):
Chainstay bridge (the supporting evidence here is pretty crappy as apparently no one takes pics of the chainstay bridges...):
Bottom bracket shells (I included this for the punch marks plus one compare pic show numbers '3' and '7' which seem to be of similar font). Does anyone know if these punch marks were a more widespread practice or somehow limited to La Perle?
Finally, the crappies pic for the end - full shot of a frame that is kind of the closest to the mystery frame (some of the earlier pictures came from this frame as well):
There were a few different styles of La Perle head badges and I would assume they had different rivet spacings (just looking at the pics online - I don’t have access to any La Perle badges). The only data point on the spacing I found is from a pic posted on tontonvelo.com where it looks like the spacing very well could be ~46mm:
Finally, re: dating - I’m not a La Perle marque expert and I don’t know if anyone took over the brand after company went under in 1955. I have seen frames that look (and are claimed to be) from late 50s and 60s, so that is a bit of a mystery to me.