I have a Parker Duofold that I use as a daily writer. It is already pretty discolored, but I am wondering if I should use a ply-glass sac instead of a rubber sac to avoid further discoloration/ Just wondering.

Ply-glass? Unless you plan to steal one from an aero 51, you may want to think about using a silicone sac.

And yes, you should use a silicone sac. I use one in any pen that tends towards discoloration. Use Giovanni's sac cement too; it holds the silicone sacs better than the off-the-shelf stuff. I also clean the inside of the sac with alcohol and dry it with a q-tip before installing on the pen.

Thanks very much for the reply. : )