Any 1951 and later Kaiser vent window would fit your '51 Kaiser. French Lake auto in Anandale, MN still had about a dozen or so Kaisers last time I was there a couple of years ago. Mike Barker or Kaiser Bill Brown
www.kaiserbill.com would likely have one too. Check Kaiser Bill's website and or e-mail him from there. He's probably got what you need.
Best of luck