Set screws on Bow rail 410 Commander

Seems like a ridiculous question about set screws on the bow rail where the rail fits into the vertical. One came out and is gone. I got a stainless steel assortment to replace it and maybe others. The 1/4"x20 starts to thread in ok but tightens up way before going all the way in. I can't imagine a metric size is right. Does anyone know if it is a specific or special size Chris used? If not I may try rethreading it at 1/4x20

Thanks

John