The wrench is screwed onto the exposed part of the vacumatic unit, then gently close the handle unit tight and unscrew the unit from the barrel.
You may need to apply a little heat to the barrel to release the shellac sealant.
If you have the a SMALL SUBDEB , JUNIOR, STANDARD, MAJOR size VACUMATIC PEN or the '51' VACUMATIC you require the RED handled wrench.
If you have the OVERSIZE and MAXIMA pen you require the YELLOW handled wrench.
BUY BOTH RED and YELLOW wrenches together save £5.00