I have to assume these inserts were for round nose bullets,
Assumption that most high power reloaders in those calibers shoot pointed bullets,
And these would be the round nose bullet seater inserts that most of us used to ignore?...
That extra insert doesn't come with a lot of seating dies anymore,
(Like pistol seating dies often don't come with wadcutter inserts much anymore)
As to origin, since you know calibers, again, we have to assume the inserts were marked with caliber???
If I'm reading this correctly, you are asking us to guess what brand of dies the extra inserts are?
What brand of dies the inserts originally came with?
What is the prize?
A question like this has to be a 'Stumper' and come with a prize for the guy that blindly guesses correctly!!!
Otherwise without a detailed description and/or picture there isn't a 'Correct' answer...
It's purely speculation on your part for any answer given, since the seating die body apparently doesn't exist, there simply is no quantifiable way to even check any answer given!
THIS IS WHY ALL EXTRA PARTS OR ACCESSORIES GO BACK IN THE ORIGINAL MANUFACTURER'S BOX,
AND WHY YOU SEE ORIGINAL MANUFACTURERS DIE BOXES IN THE BACKGROUND OF EVERY SERIOUS RELOADER'S BENCH/ROOM!
Considering the information given, and the disposable nature of the die set in general (completely forgettable), I have to guess LEE.
Cheapest dies on the market, easy to forget, and people don't keep those round die cases like they do everyone else's rectangular boxes...
The only addendum to that is Lee dies are so cheap (and I didn't say less costly) they often didn't come with any extras, like flat nose, round nose or pointed nose seater rods...
You often had to call Lee up to get the proper seater if other than 'Common'.
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If you REALLY want an answer, measure the threaded rod diameter and thread per inch count,
Note if the seater is threaded all the way up or not with a screw driver slot on the end, or has a knurled thumb screw head on the rod.
All threads on the top side used a jam nut to lock it down, and a screwdriver slot to adjust, probably RCBS.
If knurled finger adjustment, is the insert aluminum or steel?
Different manufacturers used steel, some used aluminum.
If it's aluminum & fat, it's probably Lee.
If it's steel and about the same size as the case neck, it could be any one of about 5 common MANUFACTURERS.
Just not even close to enough information for an educated guess...