At this point, I recommend taking the steel plate and flipping it over, both to examine the controller and to check the plastic rivets. Model M's have plastic rivets that are melted to hold the key assembly tight against the steel plate. These tend to deteriorate over time, so after 20 or 30 years it is not uncommon to have a few or many of these broken off. If too many are missing in any given section, it can result in inconsistent tactility or even failed actuation. If you have a lot of these rivets missing you might want to double check how your keyboard feels to see if any repairs are in order - if so you will probably have to do a bolt mod to tighten things up. Avoid this is if you can, because once you start there is no turning back, and tightening 80 or so tiny bolts can be the opposite of fun. However if your keyboard is not working properly this step can be well worth it.
As it happens, as dirty as this board was, all of the rivets are in perfect condition.