Hi Neil,
Of the 8 D4's I own only 2 have no corrosion in any of the compartments and yes both have the drain plugs intact as well as good gaskets on the access covers especially the steering clutch adjustment cover of which both had been replaced at some point. After removing the floor plates on some of my rusted machines they had a lifetime of dirt packed around the top cover along with missing bolts which can definitely lead to moisture infiltration.
When I was a kid my uncle would always say "take care of your equipment and your equipment will take care of you".

CrawlerAddict

I usually only remove drain plugs when I go through and lube the machines except for the know leakers as they get checked more often.

CrawlerAddict