Here's a WW2 era Camillus I bought with degraded celluloid covers.
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I removed what was left of the cell and rust. The blades are pretty full, and the springs are nice and strong. It appears to be the "Army Air Corps Utility Knife" pattern.
I made new covers from curly maple, and epoxied them.
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Finished with Tru Oil and Ren Wax
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Now all I have to do is sharpen.
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As long as I don't have to take the knife apart, replacing covers is easy, even without special tools.

Now when I'm at the flea markets, I'll take a closer look at knives I would have passed on because of their ruined cell.