I have 2 types of primary diesel fuel filters installed on my boat. The main engine uses a Racor 900MA turbine-type with the clear bowl and metal shield. The generator uses a Racor 230R MAM spin-on type with an aluminum bowl. In trying to determine the size and type of threads for the spin-on drain port so I can add a drain valve, I found that Racor makes the same 200-series filter unit with a clear bowl. But it's only approved for exterior use with outboards. Inboard use requires the metal bowl - which means you can't inspect it without draining / disassembling. My question is this: if the turbine type filters can use a clear plastic bowl without issue, why can't the spin-on types also use a clear bowl?