Dirty Fuel

Well.....
The 'window' screen on the bottom of the sending unit inside the tank would keep this stuff from getting out of your tank (And it plugs up and shuts off fuel to your engine!). I see you have a Walbro, so maybe you have a alternate draw straw for it. ?
What I wonder is if it was on the top of your filter and there was a way for it to get into your catch container without you seeing it was outside the seal of the filter as you removed it. To see exactly what I mean: go look at the oil filter on your engine. You can see that there is a lip on the top that is outside the seal that can catch and hold crap like this. It is almost impossible to wipe off before removing the filter. Just a thought.
Also, I don't know which filter you have; but, the center hole should be clean fuel out of your filter. That junk couldn't have come out of there without catistrophic failure of your filter. The outside holes should be where the dirty fuel goes into the filter and most are made such that it would be hard for stuff like what you caught to get back out those holes. At least very easily.
This is why I vote for it being on the outside of your filter canister (Probably on top just outside the seal). I re-routed my OEM filter drain with a longer hose and I even put a cap on it to keep the inside of the hose clean. This way, anything that comes out of my filter drain should be from the filter itself and not outside. I keep what I drain to let it settle; looking for crud and water....

Those rollover valves mentioned are 1/4" hose barbs. I don't think stuff this big is going to get into your tank this way. They do leak; but, not this badly! Also, the 'fix' tsb is to just put on about 6" of hose.... I don't care for their fix. I have pics on Picasa Web Albums - Dan - Tank Removal ...
of how I fixed mine......
I was really shocked to see how much crud was in my tank when I pulled it. I believe it was in there from the factory. I don't know for sure; but, some was metal shavings and plastic shavings. I think it was from manufacture of the truck. I think it just shows how important it is to use great filtration!
It would be nice to know where your 'crud' came from. Look it over and see what you think about my 'theory'. It would also be nice to cut open your old filter and see what was inside of it. If you don't want to do this, I would be glad to if you can send it to me. I would also be happy to post pics of what I find.
hth,