I see, you are talking about an infielder giving a sign. That is quite a different case from having discussions with a pitcher. Middle infielders often communicate about who has coverage - for instance on a steal of second - by using the sign of looking over with an open or closed mouth. And in that case, it has usually been behind the glove in order to hide the sign.
As for taking someone's word, don't take mine, just take the opportunity to watch some old games. I will be watching, too, when I get a chance - and I will let you know what I see.
Just remember, the original claim was:
ok ok i have a few seconds here....the thing that irks me the MOST (being a pitcher) is Nuke discussing Crash's pitch calling OUT LOUD?!?!?! I can gurantee you that this has NEVER happened on a baseball field anywhere on this planet EVER.
Since the claim was that "this never happened on a baseball field anywhere on this planet EVER" it takes only one example anywhere at any level of baseball to refute that claim. However, my position is that historically that claim could be refuted at the major league level not only once, but in fact that the opposite was the customary practice as recently as forty years ago.
Grist for the mill:
Fredi González, Manager of the Florida Marlins, talks with his pitcher on the mound - no mouths covered:http://en.wikipedia.org/wiki/Image:FrediGonzalez.JPG
I realize this does not match the situation where a pitcher and catcher confer on the field, so I will be on the lookout for examples that more closely match that situation.
The practice of pitchers covering their mouths with their gloves became common in the early 90's.
In the 1989 NL playoffs between the Giants and Cubs (a year after this movie was made), Cubs pitcher Greg Maddux had a conference on the mound with his catcher and - without covering his mouth - said he was going to throw a fastball inside to the hitter, Will Clark of the Giants.
Clark read Maddux's lips and knew an inside fastball was coming. He hit it onto Sheffield Avenue for a grand slam.
Pitchers started covering their mouths more frequently after that.