I know at least one person who was active at that time and has an unusual degree of knowledge and expertise in the "hardware" side of things. From his statements you bought your own rifle and if you wanted it converted you sent it in to Long Branch where it was magnafluxed and hardness tested. If it passed the conversion was done, if not you found another. It seems you could also send in a replacement bolt if for example only your bolt failed the inspection. I assume that having the people and expertise they then did, Long Branch would have lapped in the new bolt - hard to believe they would have done anything else considering the higher pressures of 7.62x51. Many shooters didn't want to take time to send in their best or only .303 No.4 and so be without it for some time and so would buy what they could and send that in for conversion, which perhaps explains the wide variety actions seen. Some could afford to buy say, a new No.4 Mk2 from Parker Hale and some could not. I would expect that Long Branch also test fired and generally ensured that rifles were set up to given minimum standard.