A little late on a reply, but I'd say your doorknob is original to your 1949 Trailwind. It is quite similar to the original doorknob on my 1948
I have taken photos to upload, but unfortunately this software now apparently no longer allows a direct upload of photos from a computer but, instead, requires a URL for photos to be posted. GRRRrrrr. If I can ever figure out how to post the photos of my original doorknob with this new improved software, they may eventually appear in this thread.
Both your and my doorknobs appear to be ordinary household doorknobs from the late 1940s, and the small curved cutouts in the door-within-a-door to clear the doorknobs when the door-within-a-door is opened separately are identical. Great to see your '49 Trailwind out on the road!