The mystery of the single crank seal solved!

A few months ago, I had Awesom-O's crankset off to clean them and I noticed a small black washer on the floor near the bike. I though to myself "Hmmm, he doesn't need a spacer on that side because there isn't one on the other side" so I didn't put it back on. A while after this episode, I had a good Google and found a Shimano instructional PDF that showed the washer should be there! Puzzled, I took Awesom-O's crankset off again and had a good look at the chainring side. I noticed that there was a fine rubber seal built into back of the crank to prevent

any moisture getting between the crank and the bottom bracket bearing and thus into the bottom bracket shell. Well, a flash of inspiration told me the mysterious washer was the equivalent of the rubber seal on the chainring side! It took a lot of Googling to find the part number but I eventually did and ordered one which I fitted back on Awesom-O (metal showing side to the crank, all rubber side to the bottom bracket) and I've since ordered a couple as spares. The Shimano part number is 20018939 and I found them on sale here.