The Westinghouse Brake & Saxby Signal Co. Ltd produced the Style 'K' mechanically locked lever frame from 1912, and a total of 35 units were shipped numbered 1 through 35, the 'L' series followed starting at L36.

It was W.A. Pearce at Westinghouse who designed the Style 'K' power interlocking miniature lever frame with its mechanical locking between the levers, and with the locking complexities of the Cannon Street lever frame with its conditional locking of signal against signal through every conceivable alternative route, that eventually led Westinghouse to develop the Style 'L' lever frame.

As far as the differences between B and K go there aren't really any other than the K usually had a greater number of contact bands because these were normally used for all-electric installations whereas the B was first designed for electro-pneumatic operation. Both had teak casing but LT companies often went for steel, similar to the L and N styles. I guess there were probably subtle changes to materials and parts between the B and K as there was a time separation between the first Bs and the introduction of the K so opportunity would have been taken to make improvements or economies. So the answer is that I've not come across any significant difference, it's more a case of marketing a "new" version by Westinghouse at the time because it's also not unknown for Ks to be used on electro-pneumatic jobs.

It was the sheer time and cost and complexity in doing a major relocking exercise that the Southern Railway ordered for Cannon Street. It actually took a month to physically fit each individual new piece of locking to the lever frame and test it before re-installing the original locking to allow trains to run normally until all the new interlocking was ready for installation.

Also another major consideration in the development of the Style 'L' lever frame was the sheer weight of the London Bridge 'K' style lever frame weighing in at 23 tons for its 311 levers. This kind of weight at 23 tons needed a very strong mechanical support for the lever frame itself to ensure that the frame did not twist or become mis-aligned at any time so that the mechanical lever locking could never be compromised and cause what is know quoloqly as a wrong side failure. 

The first table in Green, shows all the locations of all frames supplied and installed by Westinghouse Brake and Signal Co. The second table in Yellow, lists all known preserved style 'K' lever frames in the UK.