Help Specification of fixing screws for SME Tonerm connector

Who can tell me the specifications of the fixing screws for this connector socket? I lost them. I tried using M3 screws, but it was clearly not working.
Could it be some special model? Can anyone tell me the model of the screw or where I can buy it? Thank you.

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Mine measures approx 2.73mm thread diameter on my el cheapo calipers. But probably an imperial thread, either No. 3 or No. 4. Sorry I can't be more helpful.
 

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Ok, after a rummage through my spares I found some M2.5 bolts and tried one. It bit ok on the thread and could be tightened but didn't feel 'right' if you know what I mean.

So I think it might be ok with that if you're not planning to doing it too tight or constantly doing and undoing.
 
Mine measures approx 2.73mm thread diameter on my el cheapo calipers. But probably an imperial thread, either No. 3 or No. 4. Sorry I can't be more helpful.
Steven Rook, thank you for your reply. I have purchased 4 # -40 * 1/4 screws, but the pitch of the screws is different from SME's. I have taken pictures for your reference. Non standardization is a headache inducing thing.

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Dave Cawley, thank you. I have checked and found that the 6BA and 4-48 specifications are very similar. 6BA is a more difficult size to buy. I purchased a 4-48 tool and attempted to modify the screws of M3. I will report the results to everyone.
 
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