You are correct that a fire extinguisher is not a mandatory requirement, and whilst section K.3 does provide some details, you need to have a look at Section K, Appendix 3, on page 208, which gives you all the information you are probably looking for as the current standard detailed in Section K,3 is only applicable to the end of 2021. As far as i'm aware In real terms there are very few Caterhams competing in the championship who have a fire extinguisher fitted, where they were not originally race cars to start off with, where it is a mandatory requirement.