Tony
Your 20 yr old 748 should be fine if stored half properly. I have some powders much older than that including 748.
If it does'nt smell like rotten eggs and you see no rust color its good to go.
How many rds down the tube?
You say its improving. Thats common.
Might be a good time to try some of that old 748. Theres no better way to smooth up a new barrel than with a powder thats IMO midway on the carbon scale. Carbon helps alleviate copper accumulation. Too much carbon is no good either tho.
Getting some bullets to ride down the bore without leaving traces of themself behind is a good way to smooth it up.
Could be you'll find nothing as accurate as your accustomed. What has worked for me might not work for you.
If nothing else pay attention to how long whatever level of accuracy lasts without cleaning.
That lesson alone is worth the effort.
I now have three competition rifles that shoot spring,summer and fall without ever seeing a cleaning rod.
The only exception is if they get subjected to high humidity. I'll give em a dry patch to remove the green crap.
It took some effort to find the correct powder for each to accomplish that. It was necessary to have winning rifles capable of running 80rd strings without a rod.
Looking down the bore through the action will tell you nothing. You can peek down the muzzle and look for copper streaks.
Never saw a 204 yet that shot its best with copper streaks near the crown
If nothing else your chrono is telling you whatever worked in your last rifle is not going to work for this one.
It happens. Good luck