So, I've been looking at different part diagrams and it seems like there are 3-4 part numbers for about every part.

Let's take a set of floor mats...
2008 Volvo S60. #39891781: Mat set (Beige). Mat, passenger compartment floor, rubber. Set of four
There are two part numbers, at the top of the page.
If you go to the scroll list of parts, there are 3 versions of this part.
They are the same price, have the same title, and there are no pictures.

How is a person supposed to know what to buy?
I'm used to seeing part diagrams that have 1 part.
JJ

 

where are you shopping? Try this... Go to Amazon and search "volvo 39891781" and see if you get a nice picture of 4 beige rubber floor mats... There are several dealer web sites and Volvo friendly sites to shop. Some times part numbers get superceded so when I find multiple numbers, I just google volvo followed by the number and see what sites show up. Its not fool proof but works 95% of the time.

 

The parts store on Volvo's website (Owner's section).

Obviously, doing an internet search will yield results.
The question was more to which of the many duplicate part numbers should I search for?

That mat page has multiple part numbers...
39891781
39998322
39891793
39998304

Now, I do see that the URL contains the first part number.
I'm not sure if it's always that way, but maybe.
In addition, that website is missing a bunch of pictures.
JJ