I realize this was posted some time ago—but for the sake of accuracy 'n' KB knowledge, I thought I'd point out that Model M p/n 1390477 is now known to be:
• The first Model M (by any maker) to have drainage channels
• The first Lexmark-made M with one-piece caps
• The only Lexmark-made M with one-piece caps and a detachable cable
So no, it's not the same as the ubiquitous 1391401. In fact, 1390477s are rather rare. (I resist using the R-word, as it'll probably give people the idea they should dash around hysterically now, trying to find one... What I mean is, you don't see this particular p/n nearly as often as many of the others.)
Now that that's done, if you feel compelled to ridicule me for adding significant details to a topic that lacked 'em, and you have nothing better to do (for example, getting a life), hey, go wacky. Cheers, A.