That gun could very well be a screw grip. Then the slotted screw in the top lever would make sense.

I had a W&S Model 400 Grade 1 (the top grade in the 400 series, Brits trying to confuse us again!), high grade boxlock made in the mid-20's. It was a screw grip. The W&S 700 series that came out after WWII basically replaced the 400. Nice enough guns, but they cut some corners compared to the 400's. The 700's weren't screw grips.