Leaving tactics and the rest out of it. Hour record on drop bars/spoke wheels. I'm pulling this info from the Hour record Wiki page.

1972 Merckx 49.431 (pretty much trained and ate like a rider from the 50's)
2000 Boardman 49.441, (+.02%)
2005 Sosenka 49.700, (+.5%)

That stood till the UCI went to the "Unified" rules in 2014 allowing any legal Pursuit bike. Which starts to bring position and aero tech back into the equation. You would think with all of the changes in Training and Nutrition in that 40 yr period we would have seen a jump somewhere.

Interesting enough over about that same time span the Women's record on a non-Aero bike went from 41.471 to 46.065, +11%.