Reading the character in the late silver early bronze age, I never really saw her do the canary cry. Of course, the only time I saw her was when she would team-up with Batman in B&B, or the rare time she had a back-up story in someone elses book (which is what the panel below is from) . I always thought the character was pretty much just a sexy judo/hand-to-hand combat master. The first I noticed the "cry" power was in the animated JLU. Didn't realize that first showed up in the late Silver age material.
In any case, I don't think she is putting her finger to her lips in a "shhhhh..." attitude.
It looks to me like she is wagging her finger in a "No. don't even think about it" way to a bunch of thugs that just got trounced and are now picking themselves off the floor and starting to make the move to rush her (again).
The broken rubble at her feet seems to be purposefully there to imply that some serious a$$-kicking just went down.
And I gotta say, that would change what seemed to be
just a generic cheesecake pose into something that is both playfully cheesecake as well as a quintessential encapsulation of this character (or at least the way she was in the Gold, Silver, and most of the Bronze ages).
(not really a good example of the above, but a panel from my favorite BC story that does channel the playful spirit I'm talking about- art by the great Alex Toth)
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