I don't think RRA's mainspring housings are MIM. They were pretty vocal about how they were able to machine checker their arched mainspring housings, which is quite a trick to pull-off (three mixed curves are involved).
I really don't get why it would be a problem to 'mix MIM with tool steel parts', though, and I'd be a little suspect of a 'smith who doesn't want to use S&A mainspring housings. Very few 1911 makers out there don't use MIM parts and those using them (i.e. nearly everybody) invariably mix them with machined parts. Even EGW's George Smith (whose company doesn't sell MIM MSHs) has stated that "The mainspring housing is the perfect MIM part".
Too many people miss the silver lining because they're expecting gold.
M. Setter
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