According to the FAQ on Langdon Tactical web site I need a couple of "4X40 5/16" screws to mount a Holosun optics. As I have looked for them I find I need to understand the numbers.

What do they mean please and a source would also appreciated.

Are you mounting to a slide cut by Langdon that uses the C&H-type plates?

Try here for an explanation of the numbers https://www.instockfasteners.c...etofastenersizes.asp and here for the screws https://www.albanycountyfasten...5EZWkEwaAt96EALw_wcB

There are other vendors like McMaster-Carr

You may need to find a different head shape, I have not mounted a holosun.

You may need to buy a relatively large quantity. I haven't seen many places that will sell 2 or 4 and the shipping cost is likely to be high.

Thank you Sir!