[Scooper]

This seat stay treatment is often called "partial-wrap" or "semi-wrap", and the advantage is more brazed surface area, which makes for a stronger joint. Schwinn used this treatment on the higher end late eighties Greenville. MS built frames like the Circuit, Peloton, and Tempo. Lots of other builders also use this treatment in both production and custom bikes. It is not uncommon.

Curt Goodrich describes the treatment HERE in his blog.

Tim Paterek shows the partial wrap and the full wrap seat stay treatments in his framebuilding book, "The Paterek Manual".

 

[Scooper]

The bike looks like an old Batavus

Now that you mention it.

Classic Rendezvous has some close-ups of a similar vintage Batavus HERE. There are many similarities, but the points on the Batavus lugs are longer and have cutouts.