A lot of the older inserts like that were for silver soldering onto steel shanks.
I have a very old Walter face mill that uses inserts of that type. No hole and square. They get clamped in place.
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The flat top, no-hole milling inserts will have an ISO or ANSI standard designation such as SPKN1204 or SEEN43. That box is probably as lakeside53 says, a simple blank used for silver soldering onto a steel shank. I might suggest it could be a wear pad for some mechanism, but the grade CY-7 gives it away as a grade for machining steels. The "style M-168" makes it non-standard and if an indexable insert, would go into a very proprietary holder.
I got 4 similar boxes in a lot at an estate sale about a year ago and have not been able to find out what they fit. I believe that mine have a larger radius on the corners but other than that look the same. I have just resigned myself to making a tool holder for them if I ever use them.
Hi, my name is Wilson and I am a tooloholic.
They are worth more as scrap carbide than they ever would be worth spending time to make a holder and get questionable performance out of them.
Save them for when the price of scrap carbide rises again.
There is no sense of scale in the photo. We don't know if there's a side clearance angle, and if there is, what that angle is. (An SPG would have 11º)
I don't understand why people obsess about saving $2 worth of scrap carbide for use in something when you don't need it. Having a little carbide to dispose of is just a matter of collecting it until you have a couple of pounds (or a couple dozen pounds) to unload and turning it in for tools you can actually use.