Hi Fernando,
thanks for your comment! It´s a good idea to modify a hexagonal head!
Actually the diameter of the whole is 3,5mm. So there must be some alterations anyway. The head´s diameter of the surviving example is 8mm. Sadly a 4mm-screw´s head is 7mm. So I guess a way would be: Taking a 5mm screw with hexagonal head (8mm diameter), altering the head and the shaft and adapt the thread (or let a turner do this, which might be more fruitful
)
Kind regards
Andreas