There are at least 5 different 3021 versions that we identify for parts. The differences are in the Main Castings, Leg Braces, Leg Locks and Center Columns. The 3021 series is different form the 3021N, 3021PRO and the 3021X series. Older Bogen are often marked with 055, the Manfrotto manufacturing number. The is helpful, though newer 055 Manfrotto products are not interchangeable with the Bogen 3021 series.
Early versions cannot take later leg braces. The eariest versions are a single casting with parts that hold the leg in place. The later models are 2 parts that screw together. None of these are interchangable.
There are several variations on the leg braces, where the "ears" are turned in versus out, the brace is swedged on, or if there is a single bolt or a double bolt.
There are flip levers and snap levers to lock the legs. There are two versions of 3021 flip lever: single spiral and double spiral. The 3021N and 3021PRO versions use snap levers have several design variations that are not interchangeable. One version was recalled and replaced, so if you've ever pinched you thumb you'll know why it was replaced.
The center columns have single, long tubes, long tubes that can be shortened, and a short tube that has a built in extension.