Trimmed adamant full helms (in-game Adamant full helm (t)) are rewards from completing level 2 Treasure Trails. It has the same defence bonus as a normal adamant full helms. It requires 40 defence to wear.
Non-members can purchase these from other players and use them, but non-members and members alike cannot obtain them through the Smithing skill.
There is no way for players to convert normal adamant full helmets into the trimmed variety. Unscrupulous players sometimes claim that they can do this and attempt to trick other players into giving them normal helmets with the promise that they will convert them into the trimmed variety.