That is just a old Marklin-type tray not attached to the engine at all. There is a small leak on that end of the boiler and the tray catches the little bit of water that leaks out of the end cap, if that makes sense.
I was referring to your cast base having an opening where mine does not. This might have been to fit a tray to run water through the pump if the engine is equipped with one ? most don't have their trays but I think that's what the open base was for.
I think you are correct...at least, I have 2 with the hole cast in the base and one has a nice tray as a reservoir for the pump.