Just to add to what William has said. Some of the blocks had screw in freeze plugs and some had push in. My original block from ZL-1 34 had screw-in.(and it seems the original production blocks Ive seen are the same). I had an ML code corvette application block that also had screw-in.
Ive had about a dozen ZL-1 blocks (mostly non-production) and Ive seen both ways. Does not seem to be much rhyme or reason.
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