Originally it appears the coolie caps had a "S" shaped profile, hand-made, delicate, not necessarily slick, secured with a hex nut. They were notably different than the next phase of coolie caps. Here is an example of what I mean by "S" shaped profile.
And here is another data point? an ark-toothpick supposedly from 1959. Well it could be, definitely 1959-63. It has the early profile and look of the early coolie caps, and it has the hex tang nut. Notice the tear drop shape of the flutes... very attractive.
From studying photos, it seems like sometime mid- 60s, the shop started rounding off the tang nuts for decoration. I haven?t found any examples prior to about 1966. And the caps themselves became more sophisticated, more like a solid pyramidal cone rather than with the previous double-radius profile. See difference in these pictures
This rounding the nut became the default after it was introduced, presumably about -'66-'68 or so, though you could still get the hex nut if you wanted it. If I understand correctly, the nuts were rounded off using a lathe before being installed. They were rounded completely and installed on top of the coolie cap, and tightened on the threaded tang. They were not brazed or soldered to the cap. So... how were they tightened?
When I first noticed the rounded nuts, I thought that surely they were brazed to the cap and the whole cap tightened, probably by using a special socket wrench. I could see that there was a problem if they were installed and then rounded... far too labor intensive and subject to serious handle-spoiling errors when being filed down. But that is now moot.
Perhaps the round nuts were just tightened finger-tight and the epoxy held it all in place or something, along with a tap of the hammer on the tang to lock the nut. I can see why leather handles need the nut... so as to be able to tighten as the leather contracted with use.
So now from the data here, it seems pretty likely there were three separate phases for the coolie caps.
1. contoured cap with hex tang nut;
2. pyramid cap with rounded tang nut;
3. Hidden or reversed shaped tang nut.
(I understand the hidden, recessed, reverse tang nut is a way to secure the handle primarily, not necessarily the pommel. It can be totally invisible beneath the pommel, level with the pommel, or even left extended and shaped.. The pommel can be epoxied if it is above the nut)
We have also seen some terrific "tweeners" and historically significant knives such as Gary Clinton's aluminum coolie that looks to be a failed attempt by the shop to round off the tang nut after it was installed, and Gary Clinton's early bowie with the mysterious method of attaching the coolie cap, and James' very early bowie which connects the earliest coolies to the most common method used until about 1966-8 (?) or so.
Thanks for everyone's help... hopefully more on the way. I hope everyone posts their coolie cap pictures. That is a nice option.