As above, they are all different - and it is a right PITA. Sometimes the boxes will be marked, but some aren't - and then you are reliant on your wholesaler having someone with good experience and hope he can identify it.
It's a load of hassle, because (assuming you can identify the make & model) if it isn't something your local wholesaler stocks, then it'll be a special order item and delay the job by at least several days.
For many, this isn't part of the quote so they bodge it. There are a number of bodges possible, and every one I've seen is a bodge - looking like a bodge to a variable degree.
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