I bought a bunch of 'raised countersunk slotted machine screws M5 x 12' from a specialist stainless fastener supplier: https://www.stainlessmiddleton.co.uk/34298-raised-countersunk-slotted-machine-screws They fit my later type 64 bings with the two screw fitting. I notice yours are the earlier style with 4 screws. Not sure if that means a different thread or anything, but I still think it's basically the same screw. I actually feel you can get more leverage on a slotted screw in this application, than a Pozidriv head. The supplier is UK based, not exactly sure where you are, but their shipping is fast – at least within the UK!