I have a 4", .357 Magnum five shot Korth with the vent rib barrel. I've had it since the late '60s and frankly, have shot it very little.
If I recall correctly (don't have the pistol in front of me) one of the German proofs is dated "67", so I assume that is when the revolver was made.
I have a factory letter wherein Korth states that they made only 300 - 350 examples of that particular model.
And, if I could figure out a way to post a pic, I would. Ha! Got it, I think.
The serial number should start with 21xxx, it is the first series of the Sportrevolver line. There were 172 of them made in 4" and 334 in six inches. This first series lacks the roller bearing and adjustability of the trigger return spring from the outside.