F2 back weights
F2 backs were made using three different metals : Aluminum, Titanium and Steel. They look exactly the same, and there were no special markings (eg yellow dot) to distinguish them.

The lightest aluminum back, which weights about 60g, is found on normal F2s. There are some sub-variants but they all weight about the same. The heavier titanium back, which weights about 80g, is found on later F2Ts and F2Hs
The heaviest steel back, which weights about 110g, is only found on very early F2Ts. Also, it is the only back that a magnet attracts to.