I have an nylon 66 with the brown stock with white diamond, ...

I have an nylon 66 with the brown stock with white diamond, but cannot find a serial number. The barrel has a stamp on it that says KM40, and patent pending. The butt states that it was made in Ilion, NY U.S.A. It is in pretty good shape, so any info and what it is worth would be appreciated.

For about the first 10 years of production, there were no serial numbers so that puts your rifle's date between 1959 and 1968 or so.
"Pretty good shape" in my experience equates to "not really so good shape" so to me it means (if it is in good working order) the gun is worth $100 to $300 depending on how pretty it really is.