If I can direct you to the Leyland Marathon topic then you will find the information about the TL12 and L12 engines. Briefly, the L12 was a normally aspirated version of the turbo-charged TL12, which in itself was a direct development of the AEC AV760 engine. When Leyland had to withdraw its fixed head 500 series engines because of serious reliability problems then it had to find a mid-power range engine for the Buffalo, Bison, and Octopus models, so the L12 engine was offered as an option in those three models. The other option in the Buffalo and Octopus was the Leyland TL11, which was a hastily developed turbo-charged version of the Leyland O.680 engine. The L12 did what it was asked to do, being reliable, reasonably economical, giving acceptable service life, but probably not the liveliest of performers from a driver’s viewpoint in the Buffalo and Octopus. With it being a large lump of an engine L12 Bisons had a tare weight disadvantage compared with some competing 6 wheelers.