As already stated it is a late 360 balancer. Not sure what year, but at some point the engineers realized they could remove the old style eccentric weight and hollow out the same amount of material from the opposite side of the outer ring and have the same balancing effect while reducing rotating weight. So, your part is "good-er" for that engine. Double check your pistons, I don't think Silvolite//KB makes the H405P's. If I recall correctly the 405Ps are a stock weight replacement for the 360, so there should be no need to have the engine re-balanced as long as you have the offset weights on the converter or a weighted flexplate.