Prop stuffing Box Safety Collet Grub /set screws

Just renewed the Propshaft Stuffing Cord on our 88/89 2001 Ski Nautique and one of our problems was with the safety collet that sits above the stuffing nut (which has two set / grub screws that secure it to 2 dimples on the shaft) . This prevents the stuffing nut from unwinding completely off (if the wirelocking fails ) and letting a stream of water in . On our collett one of the grub / set screws had been overtightened and as soon as the allen / hex key was applied it just spun . Luckily for us we got it out using an extractor on the 2nd attempt. Only problem now is what size is it ??? I've measured it and its 5.5mm accross and 10mm long Now the age of the boat and the fact its American means it aint metric so what spec will the grub / set bolt and thread be???? Size 12 US ??? Thread pitch ??
PS I am in the UK

1/4 size 20 pitch thanks for that