2014 GL 450

I've been told for some time now that the serpentine belt should be replaced. I have a local garage says they will replace them and is ok with using parts that I supply. Best I can tell is that in addition to the tensioner there are two (2) ribbed and one flat idler pulley.

My question is, are the two ribbed idler pulleys the same or different. FCPEuro lists several with slightly different vague descriptions where they go.

Thanks, Bob c