Why are the buttons on computer keyboards called "keys"?

Key meaning (computing) one of several small, usually square buttons on a typewriter or computer keyboard, mostly corresponding to text characters, derives from the keys (mechanisms) of musical instruments according to Etymonline:

• keyboard:

• 1819, from key in sense of "mechanism of a musical instrument" + board. Originally of pianos, organs, etc., extended to other machines 1846. The verb is first recorded 1926 (implied in keyboarding).

• key:

• Musical sense of "tone, note" is 15c., but modern sense of "scale" is 1580s, probably as a translation of Latin clavis or French clef (see clef; also see keynote). Extended c. 1500 to "mechanism on a musical instrument".

In my view key is from French clé/clef and Latin clavis from the verb claudere to close. Clavis was primarily a key to a door. French clé was also used for the parts of a piano that you touch to produce a sound. In analogy used for similar parts of a typewriter. Here "key" is a kind of metaphor for a part that works like a key to a door (for opening and closing) and produces an effect as making a sound or typing a letter.

I would not derive key of a typewriter or a keyboard of a computer from key as a special sign in the notation of music as piano key or bass key. So in Etymonline. http://www.etymonline.com/index.php?allowed_in_frame=0&search=key&searchmode=none