Legacy parts
Legacy parts are parts that were removed in the VAB and SPH because these were replaced by new/better parts. Legacy parts are only available in old versions, but they do not include parts that were released in another version. However, they have been removed from the game and replaced them with the new/better parts in all craft files.
Command Modules
The Mk1-2 Command Pod was hidden in 1.4 in favor for the Mk1-3 Command Pod
Fuel Tanks
The ROUND-8 Toroidal Fuel Tank was hidden in 1.4 in favor for the R-12 'Doughnut' External Tank
Decouplers
The TR-2V Stack Decoupler was hidden in 1.4 in favor for the TD-06 Decoupler
The TR-18A Stack Decoupler was hidden in 1.4 in favor for the TD-12 Decoupler
The Rockomax Brand Decoupler was hidden in 1.4 in favor for the TD-25 Decoupler
The TR-38-D was hidden in 1.4 in favor for the TD-37 Decoupler
Separators
The TR-2C Stack Separator was hidden in 1.4 in favor for the TS-06 Stack Separator
The TR-18D Stack Separator was hidden in 1.4 in favor for the TS-12 Stack Separator
The TR-XL Stack Separator was hidden in 1.4 in favor for the TS-25 Stack Separator