sh weirdness?

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've been trying to use some external software across various of my
NetBSD machines, and have run into a problem that I've tracked down to
a difference in sh behaviour.  Fortunately, a small test case exists.

Running this

sh -n -c 'case foo in (bar | baz) echo one; esac'

on 4.0.1 returns nothing, but on 1.4T, it gives

Syntax error: word unexpected (expecting ")")

Can anyone explain what this syntax (with the leading open paren and no
closing ;;) does?  The 4.0.1 sh manpage is no help; it lists only the
no-( with-;; syntax, even though the sh it supposedly describes accepts
this mutant version.  Based on the context in which I found it, it
looks as though the case above is probably semantically equivalent to
"case foo in bar|baz) echo one;; esac", but I hesitate to just replace
it without actually understanding what the code I'm replacing does.

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