I found the serial No on the LH rear drop out by scraping the paint off.
It is 85062, with no letters, and the 8 is definitely 8 not B.
According to the Sheldon Brown and Kurtkaminer websites, which have a lot of serial no info,
it could be a Supercourse, Gran Prix or Grand sport.The latter is out as it doesn't have wrap over stays.
Looking at photos of the Gran Prix, the wheels should have large flange hubs, and this one doesn't.
It is most unlikely the person who owned the bike originally would have downgraded the wheels.