Could anybody tell me what does "4 Cors a Pistons en Fa" means? I guess it means there are 4 horns in F. But what is pistons? I couldn't find this word in my dictionary.

Could it be valve horns? I don'y know what that is, but I found it here.

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"piston" does mean valves and "cor à pistons en fa" are what English people call French horns (as opposed to hunting horns or "cors naturels"). I'm not a French horn player but the "fa" refers to the "F" side of the French horn (as opposed to the Bb side) and is what (I think) orchestral French horn parts are usually written in.

I'd take that as a valve horn in F. Dolmetsch have it as that too.

That may be how Dometsch translates it but if you are looking at a modern (as in 20th century or later) piece of music it really does just mean "French Horn."

Technically French horns no longer have "pistons" but "palettes" (I'm afraid I don't know what the correct term is in English) but French French Horn players usually use the word "pistons." A "valve horn" is only useful as a translation if you are looking at the historical development of the horn as an instrument.

I would also take issue with Dometsch's translation of "natural horn" as "cor simple." The correct translation of "natural horn" is "cor naturel" and a "cor simple" is a "single" French horn (as opposed to the "double" one currently used by most orchestral players).

I stand corrected, it's always good to have all information at your disposal especially the development and history of instruments. I was going by the terms in the abrsm theory book i've been studying for Grade 8. I'm sure they'd take both answers as correct if asked. Thanks