If it's indeed M3.5 x 0.6, it's the same size as the hardware used on the recent HP-IB connectors, the ones with black screws (early HP-IB connectors used uncolored stainless steel 6-32 American hardware). You can check for compatibility with the screws on an HP-IB connector.
This is indeed a metric size, but a very unusual one which is not easily found, even here in Europe. Very few hardware vendors have them, one of them is here :
(no affiliation with them).
I hope this helps !
Joel Setton
