Rear caliper piston size?

[Quicksilver]

There apears to be two sizes of piston for the rear callipers 38 or 40mm any way know what I have without taking apart?

[classicswede]

If it is anything like with the 7 and 9 series the higher the model spec the bigger the piston size. Base model cars with 38mm and higher spec cars like T5's etc with the 40mm I should think.
If you are changing the calipers would make sense to go for 40mm ones

[partsforvolvos.com]

one is awd and the other 2wd.

i only have a 2wd one in stock, but i reackon the 40mm is the 2wd version because the awd calipers are tiny