I think the biggest mistake that you are making in identifying your bike is this
" it's not noted in the catalog but only the Everest had a ring around the top of the seatpost, as mine has, which was to be a mounting point for a rubber seatpost boot they planned but never put into production."
This is not true.
Also, when you read catalogs, you should allow some room for things to be different than the catalog states.
The real serial number records and dates were not saved. What you read on the website is an attempt to piece the information together.
I looked at some of my bikes. I have one frame, the same as yours with the grooved seat collar and the same head tube, it has a fillet brazed bar, but its not clamp-on. I have two other frames that have the grooved seat collar but have a plain head tube.
The very large gun metal grey frame in this thread is an Everest frame.
You should look for a fillet brazed bar for your bike.