Working on resealing oil leaks and a major leak was the timing cover. I removed it to discover a loose timing chain, but what really surprized me was the cam gear has nylon teeth! I was told the engine was original, or at least the heads were and to be sure and use lead additive because the valve seats were not hardend inserts but milled original heads. I was thinking that nylon gears were not introduced until the late 70's, and only GM until they figured out they were prone to breaking ect. Am I correct to think that the nylon gear means the engine is possibly a replacement? Nobody would replace a timing chain and use a nylon gearset?? How would I determine the valveseats are original without removing the heads themselves? Is there a serial number under the valve covers much like my old small block ford? This is my first SBC to ever get my hands on.

Thanks RK

1959 Apache swb stepside
Atlanta Ga.