How do B-2s have "tail" numbers? :-)
It's a generic term...Spirit of Alaska's is 90-0040. Or, in Air Force aircraft maintenance lingo, "Balls-40." I had the honor to see the first 8 operational aircraft being delivered to Whiteman between 1993 and 1995. I also happened to be at Whiteman on leave the day Alaska was delivered in July 1996.
And, I hate to say, I was not surprised when I was told Spirit of Kansas (89-0127) was the one that crashed in Guam.