Commercial pre-war High Power pistols with the fixed rear sight were built in the 9,000 to 12,000 serial range, according to Vanderlinden, FN Browning Pistols. They would also exhibit the slot for the shoulder stock. All other pre-war officer's models came with the tangent sight.
The one pictured above seems most likely to be a post-liberation gun. Vanderlinden does not seem to offer a good serial range for them.
Here is the pre-war Belgian contract officer's model with tangent sight and stock slot.