For those that aren't aware, the standard C5 toolkit is more useful than most people realise. The black tube itself is actually a "jack" - when placed under the bodywork it will keep the rear wheel off the ground, a real help when fixing punctures etc.
More importantly, this spanner is more useful than most ;
The "open" end is for removing those odd sized caps that hold the original battery top to an original Sinclair battery;
..and the "closed" end is for holding the bearing collar still on the offside (brake side) rear wheel whilst you tighten the wheel nut. Prevents you turning both at the same time and overtightening the bearing collar ;