Patsy at Trac Mac has a brake pad reference book that looks up the BMW model name and year and then find the code. The rear from what I understand are thinner than the front which can make sense as almost all the braking is done on the front wheel when pulling hand brake lever, yes I know it is a around 80/20 split with the back. I almost never use my foot back brake but sill get kak mileage on the back pads.
I didn't mean rear versus front. I cannot remember the details, but SBS (I may remember the manufacturer wrong as well) makes rear pads. They are identical, but one set is about 12mm and the other about 7mm. This happens because the shape of pad works on more than one bike, but the one bike (in this case the GS) callipers opens wider and can accept thicker pads.
I have had a situation where I wanted to buy two sets. Needed to change both our 2009's rears prior to a long trip. Went to one supplier and they only had one set, so I bought it and went to another. Bought a set there at half the price. I foamed around the mouth all the way home because the first okes ripped me off. At home I felt the one set was lighter than the others and saw the difference. I think what some okes do, is they know about this and if they don't have stock will sell you the one that will work.
My current set of rears have reached 20 000km, which is a good 5000 better than I have ever done. Goldfren. No idea why the GS' chow rear brakes like they do.