And here are some more gut shots.
With the modular assembly, the only unique feature of the multi-deck switches are the center shaft and the screws holding it together. Back to Ian's original question, it would be very hard to accurately shorten the center shaft to turn a 3 deck switch into 2.
2-pole deck. Note normal position of the contacts relative to the center shaft. Probably doesn't matter, but this is how the factory ships them. Those little contacts like to fall out. If you need to put one back in, the nub points to the rear where it makes contact with the deck itself. Non-shorting contacts shown. I don't have a shorting switch open right now.
Deck itself installed over the contacts.
Rear cap is just the plastic part of another deck. End cover goes over this.
Hope this helps. Seems like this isn't new information for many, but it's good to have a reference.
**EDIT** Note: I lost all of these images off my server and don't have them anymore