Era Bill most have a two number date stamp on the top mount, dark are the oldest around 1977, I have a set of light grey which are stamped 79 & silver are last of them made, shaft length was the main changes over the models & small valving changes from what I have read, also have a full service manual on CD if you what a copy, very easy shock to service & once setup correctly are a lot better than most of must have brands.