/6 Valvecover

There are three Slant-6 valve cover bolt patterns:

1. used on 1958-1959 prototype engines. Almost nonexistent even when new, they are a less-than-once-in-a-lottery-ticket sighting these days.

2. used on 1960-1980 production engines. Four different valve covers, all bolt-on interchangeable.

3. used on 1981-1987 production engines. One kind of cover.

What's caused the confusion here is that while the '81-'87 hydraulic-lifter engines all came with the final-style valve cover as shown in post № 7 of this thread, the pre-'81 cylinder head bolts right onto the '81-up block and accepts the hydraulic-lifter valvetrain components. The newest Slant-6 engine installed in a new production vehicle left the factory 3-1/5 decades ago—plenty of time for parts swapping to have happened.