Steering rack bush question

I notice a number of people replace the old plastic bush with modern material and then fit a phosphor bronze bush right next to the plastic one ... Sounds a good idea to me but it will then cover the hole that a plastic (filler?? ) screw in it ...
My question is ... What is that plastic screw for and if it's to top up the oil then puting a second bush in does not seem a good idea ( as it locks the hole )

Comments please