Standard Housing Spanner
The housing spanner is a tool to assist with the opening of the filter cartridge housing. As water fills the filter housing pressure builds up inside the housing, this fact plus age can make the housing difficult to open by hand. The housing spanner allows an individual to get a better grip on the housing and so makes it easier to open the housing.
Please keep in mind that the housing spanner is purely for opening the housing, do not use the spanner to reseal the housing as it can result in "Over tightening" which can damage the housing's thread leading to leakage and making it very difficult to reopen.