I wound up with all of the material on the Involution vinyl twofer via combination of Hill's Change and the BN Rivers CD Dimensions & Extensions. Being late to the game and generally too cheap to spring for Mosaic big boxes, research was necessary...
There's some great string quartet material (but no Rivers) on the Hill Mosaic Select #16.
Looks like I am mis-remembering the twofer recording - I had thought the string quartet arrangements were with
Sam Rivers, but I guess they were the tracks of
Andrew Hill Quartet, without Rivers. I found
Change on Spotify, and it is the four tracks from
Involution with Rivers, but not the tracks I enjoyed the most, which were the string quartet sides. I have that Mosaic box and need to get it out and listen to it.
Andrew Hill is just about my favorite jazz pianist/band leader.
EDIT: I Googled "Andrew Hill string quartet" and found the twofer I was really thinking of,
One for One:
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