I wonder if your using a metal detector? Cause you should be finding some cool old buttons & no telling what else being your finding all the inserts out of them
If you would take sometime & look this pdf file over.http://www.jefpat.org/diagnostic/SmallFinds/SleeveButtons-Cufflinks-Studs/Linked Buttons.pdf
You'll see what these type inserts would mount in & a date frame of when they were made.To find as many as you have I can't figure out why your not getting other cool metal finds as well.You need to hit that site with a detector.
Take Care,
Pete,