Some time in late '07. I was made aware of this by someone else. He had 4 or 6 that weren't right compared to the rest. Thankfully I had a few of the correct ones to replace them with. In all total I sent back almost 200 pieces. The way you can tell is by the adjusting screw. Correct ones have the screw pretty close to the end of the rocker. The wrong ones have the screw farther away from the end. Some of the early sets have both types. People have used the wrong ones with good results, but these were on basically stock type engines. On those we worked together to come up with creative ways to get them to work with no problems by making some mods. So far there have been NO recurring problems. Recently installed a set that was bought & never used. Installed them, made some mods & the car went faster ET & MPH. Tom T.