Again, this is just general information gained from experience in the trade. Not so important on passage doors but it is on chests and safes with surface mounted locks (and boltworks), enclosing the lock mechanism in a case prevents accidental jamming of the parts by anything held inside. Of course, the design of the lock as well as what is held within has a bearing on how important enclosure might be. A locksmith’s chest from the 1600’s would contain lots of small parts and tools which could easily foul the lock, if the mechanism were exposed. Regarding the terminology, I can’t do any better than hook bolt. Older bank vaults with sliding grill gates used them. In some of these older chest locks, I see as many as 4 hook bolts used, sometimes tapered, sometimes not. It appears that the “crablock” style is one of the simpler 2 bolt types where the barbed catch would often be anchor shaped. Doug