The .40 hammer pictured came from a Model 411. It is not MIM.
It was used on .40 pistols until it was replaced by the MIM hammer which was the service replacement for all blue spurred hammers, as best as I can tell.
Your hammer is the earliest "Performance Center" hammer referred to in the article.
The hammer I pictured came next. It would seem the company figured out how to make them lighter by drilling a hole and thinning the spur and without having to mill that slot in the rear. Then came the MIM hammers and they were light enough and plenty strong for the job.

John

P.S. I did not have an early hammer to photograph. Thanks to BLUEDOT37 for providing one.