Metal Injection Molding - a modern (cost saving) manufacturing process where a slurry of metal particles is injected into a mold, put under extreme pressure and heat treated. The result is a part needing very little machining. Used for hammers, triggers, other small parts that are usually not subject to heavy stress.
According to Std. Catalog:
Model 10-10 introduced 1988, in 1997 some MIM parts added.
Model 10-11 introduced 1997 full compliment of MIM parts.
Model 15-7 introduced 1994, in 1997 some MIM parts added.
Model 15-8 introduced 1997 full compliment of MIM parts.
(by full compliment I mean whatever MIM parts were going to be used were fully in place, the number of actual MIM parts is relatively few. Mostly used for curvy, complicated hard to machine parts - like hammers & triggers)
One easy way to tell on the guns I own is look at the back of the trigger - if it looks like a solid piece of metal it is pre-MIM, if the back of the trigger looks hollow or concave it is MIM.