I have an '86 and an '87 elance 400 - both have the same decals that yours has. They are the blue ones, though - but I think black was also one of the colors. My '86 does have the silver head tube. The '87 has a blue head tube.

I, too, wouldn't worry about apparent inconsistencies too much. During this time, Trek used this particular frame for a lot of bikes (for example, '86 400, 520, and 500 all have the same frame) - 531 main tubes, cro-moly fork and stays, and the same slightly relaxed geometries. I can see all sorts of things getting switched around when they finally get painted and built-up - and I doubt they were religious in documenting that serial number XXXXX actually became a 400 instead of something else. I could see the same things happen with paint, decals, etc. Unless you have reason to suspect this is a fake Trek (and I doubt there is enough money in this for people to go to much length to fake Treks), I wouldn't worry about it. The seat lug should have Trek embossed in it...so, you know it is a Trek.

You could check the components of this bike vs. what you think it is (and similar offers from similar years) - provided it has original parts. I think these change more, year-to-year, than do the frames themselves. You could also measure the lengths of the tubes - to see if they match up with what the catalog says. My guess, though, is that the bike is good. Enjoy.