Afternoon Ady.
You've sort of answered the question yourself.
The first three numbers/characters are the model that the part was first fitted to - if subsequent models used this part, it won't change.
The next five determine the type of part - speedo, tacho, exhaust etc. If a part is one half of a two part fitment, or one is for the left side and one for the right, then the numbers may well only have one character different (eg left and right silencers).
