That's why the original owners only fitted them when they were actually using the vehicles!
'Inter-metal corrosion' - Electrolitic or some such scientific term, reaction between the Aluminium periscope body and the steel of the turret.
I clean them up, apply 3 or more layers of paint, allow to dry thoroughly, coat the 'joint faces' liberally with
PURE silicone grease (Anything petroleum based, Vaseline, etc. will 'eat into' the rubber gaskets I hope you kept!)
to keep water and air out, and refit.
:cheesy: Purely by coincidence, I have a few periscopes available, at £55 for a set of 3. Obviously used condition.
PM if interested
'Chas.'