Which filler scew?

I have a S&W Model 28-2 with a Leupold scope and mount attached, the scope base was mounted in the late 70's. I want to remove the scope and reinstall the factory sights. What size filler screws do I need to buy? Anything I should be aware of to remove the base? I plan on using the gun for CMP Service Revolver matches.

Thank you

I believe the Dual Dovetail Leupold base for the S&W N frames used 6-48 screws.

I have L and K frame revolvers and the scope mounting screws are under the rear iron sight mount. They are usually drilled and tapped, open the cylinder and look under the front strap for 3 holes. The middle and rear holes are 6-48 and the front one could be the same otherwise 3-52. If you have your rear sight it should have the proper screw with it.
Stan