Different ejectors for the 9/.40, and the ejectors are molded into the sear housing blocks (meaning there are 9 & .40 sear housing blocks).
Also, as I recall (without getting out my armorer manuals and notes) the trigger bar guides used in the 990L and standard 99 models were different dimensions (part of the sear housing block) because of the difference in the DA/SA and "QA-type" design and the firing pin "timing" needed for each model. The timing is especially very important for the standard 99 model, because it makes sure the release point for the DA and SA trigger pulls don't interfere with each other (which might result in light strikes in DA mode or possibly cause damage to the firing pin and firing pin safety block due to the wrong "timing").
Also, just changing sear housing blocks may become problematic, as Walther changed the inside dimensions of the frames at one point and the sear housing block dimensions changed accordingly. I ran into that when having to replace a sear housing block due to a broken ejector once (wrong caliber ejector had ended up a gun, somehow).
FWIW, they also later on changed the design of the blocks regarding the trigger bar guides for their 99 series guns, from the early fixed post (of potentially different heights) to an adjustable lever, and they are not interchangeable in the frames for which each was used.
