I was playing around with a couple of bolt bodies for a No4 Mk1 and found something interesting. I have three bolt bodies and two boltheads, a no2 and a no3. One bolt, the one belonging to my rifle closes tight on an empty casing with a no3 bolthead, another closes tight on the same casing in the same receiver with a no2 bolthead. If I switch the heads i can feel a fair bit of play in the bolt belonging to my rifle that now has the no2 head. I will have to do more measuring tomorrow but one bolt body is definitely longer.