This camera was an early "Baby Boomer," put to market soon after the end of WW II. It was in production from 1947 until 1952 until its successor, the Model C4.
The Model 21 Markfinder utilized image targeting technology developed by Argus during the war for military optical devices.
But it has no rangefinder. You must first determine the distance to subject by other means, then manually set the dial on the lens for focus.

I put a lot of work into restoring this example, s/n 0000022508, and did a rather thorough job of cleaning , repairing, and calibrating both camera and its lens.
When I was done, I could not resist the urge to take it out for a test run.

Film was Kodak ASA 200.
First 2/3 of the roll was shot on a hot day at Claire Gempp Davidson nature preserve in St. Louis County.
End of roll was another hot day at Pleasant Valley nature preserve in Jefferson County.

The negatives just came back from Parsons, Kansas, in today's mail.
I thought the Kodak film showed a bit of grain.
The camera and lens did well enough for a 70 year old camera. Images may not be as sharp and clear as with an Argus C4. But they are mostly adequate, and exhibit a different feel, a slightly surreal look maybe?

Please click on thumbnails to see enlarged.
All images are ©2018 E.J.Kowalski.
Thanks! Ed