I bought a box of pink Janome bobbins and a box of blue supposedly so I could tell apart the cotton and poly thread (but you can really tell just by looking at the thread).  Anyway, the pink have the J and the blue do not.  I put them behind my back and felt them to see if they had a different "flex" or if I could tell them apart by how they feel and I could not.  They look the same except for the J and they both work the same.  I've never had a problem with class 15 bobbins and have an assortment dating back to the 80's. After the discussions about the so-called superior J bobbins I went through my collection and found a few with the J but most do not have it.  I even found one with a "9" or "6" on it.  I have a prewound pack of Janome bobbins that don't have the J. Some do tend to have a very slight curve towards the center and others do seem flat but they all seem to work well.  When I first had my MC6000 (New Home) in the late 80's I even used metal bobbins in it before I found out several years ago that this was a no-no.  I would never put a metal bobbin in my 15000 but I never had a problem with the 6000 and it still runs like a dream (but no more metal bobbins!)