In looking at pictures on the Darne link, and in looking over catalogs, etc. that Darne mailed me years ago, and based upon my studying of the Darne's I have owned, this is my assessment:

Look at the pictures, Darne link, of the carbine express pictures (they can be enlarged tremendously, for close viewing). They show the Doll's head, for 39R Blaser, 7x65R, 9.3x74R, double rifle, and a combination gun too. The action itself slides forward onto the barrel doll's head & locking bolts in action frame grasp this doll's head on both sides, sliding into the doll's head recesses on each side, making for a much stronger connection than is possible with their earlier barrel bolt as used on the milder 8x57R. Clearly, they developed this doll's head for the heavier calibers, and for the calibers that produce more intensity breech pressures.

Scroll on down this same picture page of carbine express Darne site, and you will also see good picture of the earlier barrel bolt system on their 8x57R. It is same system used on their shotguns, or at least on the ones I have examined through the years. As action slides forward onto barrels, a round barrel bolt slides into mating hole in action frame, and is locked in place AT ITS BASE ONLY, by a side sliding round cross bolt. My opinion is that this "barrel bolt" system in no way would be as strong as the common Greener crossbolt that we are so familiar with, OR AS STRONG AS THE LATER DESIGNED DARNE DOLL'S HEAD BOLTING SYSTEM, and explains exactly why they designed their doll's head bolting system.