Ask any classically trained double gunsmith and he will tell you that these doll's heads, Greener cross bolts, Scott bolts, etc., exist primarily for psychological reasons. Virtually none of them are fit up to engage surfaces for locking.
In plain language, they were fashion-of-the-era sales gimmicks. And as Dean points out through the Parker Trojan example, generally unnecessary.
The one exception I can think of offhand is the original Westley Richards A&D box lock doll's head. It engaged a sliding top bolt to lock the gun... not the frame. Later, the Purdey double underbolt was added, which made the top bolt superfluous .
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