why do these 2 o2 sensors look different? they are suppose to both be NTK but the one on the bottom is stamped NTK Japan and has slot openings on bottom section while top one has only Japan stamped with half moon slot openings on bottom section. i have seen NTKs at the website with half moon slots. is one type for right and other for left? which would be which? or did someone just send me one that is wrong and which one? just would not like to put two different o2s on my car. thanks jason.