That's a Panzer III (Mark III) with a 50 mm short gun (50mm KWK= Kampfwagenkanone)

Just some trivia: The Germans actually designed the Panzer II (20mm Gun) and the

Panzer IV (75mm) before the Panzer III. They had the I, II, III, IV concept ready but did

not develop them following strictly the number. - (description contributed by Spider)