Let's see if I can make this locking thing clear.
People are using the word "unlocked" in a generic way to mean "enable", so it has come to mean several different things for Athlons.
Originally Intel "locked" the multiplier on their CPUs, so that it was impossible to reset the multiplier at all.
Tbred B's, and I guess now Bartons, are unlocked in the original sense. There are bridges on the chip that connect the multiplier circuitry on the chip to certain socket pins. Then your motherboard could set any multiplier, provided the mobo has the circuitry and the BIOS has the settings. (And I think that KT400 mobo should have the needed circuitry.) The problem was: quite a lot of pretty recent mobos did not implement the ability to set one out of the possible five multiplier pins. (Older Athlon's had only 4.) Therefore certain multipliers were not available. If the CPU came with multiplier of 12.5 or less people thought it was "locked" to the lower multipliers, and if it came with a multiplier of 13 or more, it supposedly was "locked" to the higher multipliers, although the CPU was really not locked: the mobo was deficient. Depending on which set of multpliers you want, there are mods that you can do to the CPU, or the mobo, or the socket. What they do is set the multiplier fifth bit.