It looks like a housing for an added spring used to keep the erector tube pushed against the adjustment screws. It is a added modification. It is not a feature manufactured by Leupold.
That model is considered obsolete for serious benchrest competition but it is a good ammo and rifle testing optic.
While the scope is not often seen on this side of the pond, it is a very well made addition. A bit of sanding to key the surface and gloss black paint would make it more sellable to many possible buyers.
+1 on the test scope idea too, but I use one of the steel tube fixed power Weavers. These old scopes can hold rock solid zero's. Their best feature is the AO front ring. Gives much more precise parallax adjustment than the little side focus knobs.