Of course the stock filter will act like a water trap. The problem is no visability and you can easily accumulate more water or garbage than it can hold from a batch of fuel somewhere. There is not even a drain as you mention. The separate water separator should vastly extend your present filters life as well.

I also do not know the micron rating of the internal screen in it. It's primary function just might be to keep the larger junk and water in the separator. Reducing turbulent activity stiring up the contents as well in there.

I also have to wonder if people are aware that water through a piston type injection pump is like running sandpaper through it. Or the fit in there is so tight the metal can gall and tear. Even if not that extreme it still is not good and will shorten the lifespan. Thats why I could not understand the elimination of a true functional water separator, visual inspection of fuel content, and drain provision from mercedes . Just about the most stupid thing I could think of and they had to know better.

This is a pretty comprehensive separator even having what appears to be a remote drain location for a tube included rather than dumping the drain contents into your engine bay. I also remember from the past the two bracket mount and rubber isolator or anti squeek rubber. Thats why I am leaning towards old stock. I might be all wet here.
My problem also is on occcasion the bulk fuel carriers have not cleaned their tank out properly before loading fuel. So the problem might be visible in the transparent water separator. . I do not know the quality but a vendor has been selling one or two on ebay per week for about 16.00 by the time most transactions end. I did purchase one but not delivered yet so unsure of quality.
Quite a hardware kit for it including a mounting system in the photo. No filter to buy just remove the stainless filter sleeve periodically and wash it if required. Seems to be a cylinder with a top and bottom cap. Drain on the bottom of course. What I do not know is if the cylinder portion is glass or plastic. Plastic will yellow with age etc. The configuration indicates it is more likely glass. If plastic my thoughts are it would have had a drain in an incorporated moulded base type of thing. He specifically lists it as for 123s. But probably universal.
No brand name evident in the ad either. I will comment more as soon as it arrives. If anyone wants an item number on ebay to have a look I will post it in a few minutes as soon as I find it. Guy has 100 percent feedback and the only downside is he charges 11.50 for delivery inside the states and 18.00 to Canada. I did not find other water separators in this price range. In fact the bulk were combination water separators and filters with some visability of contents.

Okay then the item number is 270234550983 this item finishes on thursday/friday three days from now. He always puts more up so try not to drive the price high.: I believe he has either a lot of them or access to many more. I wonder if it is an old stock item or who the maker is? Since you do not have to replace a filter element it is somewhat irrelivant. I looked at other current offerrings and decided I liked the washable stainless filter sleeve. Also thought visability might be better than with a filter cartridge in there.
Reflecting carefully on the above. I may always have been cheap, but always told the women I was not easy.

I also wonder if we should try for a bulk order? Lets see what the quality is like first. I have also requested the vendor to list one with a buy it now price as well for your benifit if there is interest. Anything to prevent bidding into the stratosphere occuring.