What was Walter Matthau's real name?

According to the Social Security Administration, it was Walter Foghorn Matthau, source of mystifying marine middle moniker unexplained.

According to the World Almanac, it was Walter Matuschanskayasky, which agrees with the actor's credit in 1974's "Earthquake".

According to various other sources, it was Walter Matashansky... or Walter Matansky.

So which one was it?

A rather pedestrian Walter John Matthow. The actor loved spinning tall tales about himself. And no, he wasn't trying to hide that he was Jewish. Not with those fake monikers.