At that time, I actually had an ownership interest in a travel agency. There were frequently business travelers who had negotiated with their clients that travel had to be booked in K or above, just so they could get the upgrade. My memory is fading a little, so eventually there may have been some inventory controls, but at the beginning, there certainly weren't. Buy the K fare, and up you go. But as you point out, the only lower fare was an L fare in that era. Now we have L, U, V, T, X. (Not to mention E). I guess there was a good reason for it, but I think adding another half dozen fare classes didn't accomplish much other than to make the system more complex.