I think the change from screws to rivets occurred around 1952-53 I have owned 49-52 both Chev and GMC. For example the 49 GMC used very small clutch heads while the seven 49-52's I have owned all used a fine threaded phillips type. I know this has been discussed here before I would be interested to see what others have found with their trucks. I think someone also posted on the tool they used to set the rivets so it may be worth a search.