If it's MIM, there should be some sort of mold or gate mark on it usually on the striker face. The S anotates stainless steel, and is usually found on investment castings to keep from getting mixed up with steel parts. Sounds like you have a cast stainless steel hammer. Probably was machined, or precision ground for the width dimensions.
PS: Even if it was MIM, it is possible to MIM stainless steel these days.
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Really interesting....Don't ya think??
PS: Even if it was MIM, it is possible to MIM stainless steel these days.

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Really interesting....Don't ya think??