On the later cars, guys in some Jeep posts on other sites like the LH manifold on the '75 Buick Skylark. The dump port comes down near the middle with some reverse slant. That kind is preferred to the type with the exit all the way to the back. If the port is too far back, they report interference with the brake cylinder.
That first V6 231 manifold was part no 1259129 which is cast into the outer face. It was used on the following Buicks and other GM cars:
It was also backwardly compatible for use on the '66-67 225.
The next manifold used was the 25502475. Parts listings note it was to be used with spark plug heat shields. Cars using it were:
'79 196 and 231
'80 early 231 and 252
The next one was the 25507801 for use without spark plug shields. It had an EFE actuator on the manifold. Its usage was:
'80 late 231 and 252
'81-82 231 and 252
'83 early 231 and 252
Next came the 25516359 which did not have the EFE actuator. It was found on:
'83 late 231 and 252
'84 231 and 252
The shapes of these last manifold numbers is not known but may be available from Internet photos. This list does not include fwd or turbocharged cars which generally use their own part numbers.
What has been, can be again. (Bob Wills, 1942)