other way if i understand what you say correct the end with black rubber seal goes in the housing first as there is a seal in the end of the ousing that you took off
my reckoning its in the wrong way round as i said earlier
you would also do well in putting the presure gauge after the pre filters as you ll measure the drop more consistantly of the flow over them and be able to gauge when filters need changing and see the drop in presure.
the presure gauge serves no purpose where it is now, but after it will help you see whether you need booster pump or when filters need to be renewed.
regards tony