This small flexible plug is an interface between the SendIt level and the Brandt Built Ocular Housing. It allows the fiber optic cables to be treated like one large plug instead of trying to put the 3 cables into the Brandt housing. If you take the cover off a lot to replace batteries, etc, this become very handy.
 
It also is tight enough to hold the cables into the housing better than without it. 
 
Just one of those little conveniences that isn't necessary, but sure is nice to have. The Brandt Built Ocular Housing has been one of my favorite additions to my rifles and is still the best leveling combination in my opinion. 
 
WARNING: There was a run of Brandt Built housing units that seem to have much shorter tubes than most of them. If you have those, this may not have enough area to stick onto.