Well, I wasn't going to explore this any farther until the weekend, but now my curiosity is piqued. After examining the serial number outside, I'm pretty sure the middle two digits are 86 - so the full serial is (I think), 558624. There is also a decal on the downtube (the one the seatpost fits into) that says, "Double butted tubing - High Manganese density". I've take a couple of photos, and would be glad to email them to anyone interested. They're a bit too big to post here.