Nylon cam gear years

[ASPEC]

Good Morning,
Does anybody know in what years and in what GVW that GM used the nylon cam gear in the mark IV 454? Searching yielded no definate answer.
Thanks in advance for your help.

 

[edgephoto]

I just replaced the timing set on my 1987 GMC. I have owned it since 1988. It had nylon teeth on the cam gear.

 

[bucket]

My '91 454 has the original nylon cam sprocket still.