The blue circled bit on the diagram is the securing strap for the extingisher that goes round the bracket
It would have been part of the kit from new but I don't think it's essential as the extinguisher is normally held in by the bracket. The strap was just an added precausion to prevent it being dislodged accidently.
You could always try and find something suitable that would do the job if it's missing
Russ