This weekend I was using my PEC 6" hook rule which also has a sliding 118deg drill point gage. No Starrett, by any means, but a pretty good tool nontheless. When I bumped the hook against something I noticed some movement. That caused me to take notice of the 'empty' hole thru the U-shaped bracket that holds the hook onto the end. "Hmmm", I wondered, "What happened to the screw"? Well, after wasting an hour or so looking thru the drawer where it had been stored, as well as all other drawers where it 'just might have been...' at some time in the past, I just happened to spot a small black screw-like thing right smack in the middle of one of the few bare spots on the floor. Eureka! When I picked it up I could tell by the nice black finish and crisp slot in the head that it was the missing item for sure. But wait!... whar's them threads? What? No threads? How's that thing work? Now I'd already noticed that the holes in the bracket and the end of the rule didn't seem to line up precisely when the hook was in place, so the light came on, and it dawned on me, ...and sure enough, when I looked at the screw thru a magnifier I could see a slight eccentric cam in the center portion. (The screw's only about 1/8" long and maybe 3/16" in dia.) After studying and fiddling with all the components for a few minutes, I stuck the screw in and gave it about a 1/3 turn. ...And sure enough, it drew the bracket up tight and locked the hook in place nice and rigid.
Now I'm sure most of you folks encounter clever devices like this probably a half dozen times before breakfast every day. But, tho I've seen cam locking applications before, this was the first time I'd seen one like this. And I thought it worth mentioning, just in case there's one other poor soul like myself out there, who has led a similarly mechanically sheltered life.
Since this is my only hook rule I can only guess, but I'm sure Starrett and B&S and others use the same approach to attach that hook. I guess another reason for posting this is to proclaim my total respect and admiration for whoever came up with that idea for that tool. It is a beautifully ingeneous application of that cam concept!
Starret does indeed use an eccentric as you have described to retain the hook on their hook rules.
When I started going to Machine Tech. classes, there was a table with a few old, rusted tools that were going to be tossed if nobody wanted them. One of the tools was an old 8" Starrett hook rule which was badly rusted, but still usuable after I cleaned it up. Sure enough I spent quite a while wondering why the 'screw' would turn either way but never loosen!