As with all Luger parts, (and most other firearm parts for that matter) the screws used for the grips were made to a standard...
If you have grip screws where the slot size vary, then at some point in their life, the slots were modified by some owner by being cleaned up with a "swiss" file to eliminate the burrs caused by using the wrong size screwdriver...
Here is an extract from the 1913 military blueprints that details the slot to be 10mm long and 1.1mm wide... 1.7mm deep. The slot is NOT tapered, but straightwalled.
Anyone desiring a complete set of these blueprints on CD, the domestic price is $25.00 including first class postage and packaging, and the international price is $27.00 for international air mail. Ordering instructions are at the top of the FOR SALE forum. These blueprints are a must have for any serious student of the Luger Pistol.