What mechanism did the turbine reversers of early high-bypass turbofans use?
You're asking about turbine (hot) reversers of early high-bypass turbofans.
747


flickr.com and twitter.com

It had the same mechanism as the cold bypass (blocking doors), similar to what the C-17 (right image) uses nowadays.
L-1011


Lockheed L-1011 TriStar - Airliner Tech Vol. 8

It had target-type, but only during development.
DC-10


Design Features of the CF6 Engine Thrust Reverser and Spoiler (sae.org) and airliners.net

It had– &quot;something else.&quot;