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It was a Buick engine. GM divisions used be decentralized & develop their own stuff as a result of purchasing existing auto brands. Until the 80’s engines especially weren’t shared much, & that V6 had been around since the early 60’s in some variation, so GM didn’t “give” it to them, Buick developed it. They would share “platforms”, like Camaro/Firebird, or the ones you allude to, but the divisions were able to design bodies & interiors as well as engines. Buick, Pontiac, & Oldsmobile, for example, all had 455’s yet they shared no components. Later into the 80’s & 90’s the standard became to share common engines. Ford/Lincoln/Mercury & Chrysler went this route much earlier than GM.It's strange that GM gave the hot version to Buick. The Riviera is the same car as the Cutlass, LeMans, or Monte Carlo. Would have been more on brand for the Pontiac or Chevy.