It's the dent in the mudguard from the single saddle that ***** me off, as well as the metallic clank, GB. Next time I build a Plunger rear, I think a 1" O ring around the top of the fixed column may be enough to act as a crude bump stop and stop the clank without limiting the suspension travel too much. That it actually clanks shows it's moving, plunger columns parallel in all vertical planes can't be common, they reckoned internal friction was the damping medium.
That top spring pictured looks to have a homespun spacer to raise the ride height, and like Rex says, the lower spring is in full compression and the damper is not really worth the effort, good chance it also fouls the lower mudguard stay. Rusty part is the sliding column, exposed workings pick up water and grit and if you go for this style it will end up as a rigid and a bugger to dismantle.
Swarfy.