I think in both cases it is the quality of the original that is causing the problems, neither of these are the in house artwork.
The Hewson drawings have been reduced down to A4 from the originals which were probably A1 or maybe A2, Looking at the actual printed mag the smaller type is not that easy to read anyway.
Similar with the radial, this uses the original hand drawn ink drawings, which look to have been reduced by 50% to get them in the mag so a few more file type conversions, cleaning up the "dirt" from the tracing/dyeline, etc will have degraded the images.
If you compare these drawings with say the Darjeeling in the next issue 4405 which is in house artwork you will see that they reproduce perfectly well so it is not a problem with the quality of the actual archive but the artwork that was used in the original printed mag.
Where commercial drawings are available it is always best to use the ones in the mag as a guide and get the real thing if you are actually building.
J