What this sentence is saying is that the only pipes in this organ that are not made of wood are the last (top) 15 of the 2' Fifteenth stop (that's the other question, which for the moment I haven't answered in case Taña Dalglish decides to convert her reference entry into an answer) and the whole set of pipes of this stop, the 8' Regal.

A "registro" is called a stop ("an individual voice in the organ, composed of one or more ranks of pipes"). See for example this previous answer I gave:
https://www.proz.com/kudoz/english_to_spanish/music/6195925-...

An 8' (8-foot) stop is one that sounds at "unison" pitch: the note you play is the note you get. The name refers to the approximate length of the pipe(s) for the lowest note. The shorter the pipe (and therefore the vibrating air column when it's played), the higher the note. As with a string, half the length means an octave higher. So a 4' (4-foot) stop is one that sounds an octave higher than the note played, a 2' (2-foot) stop sounds two octaves higher, and a 16' stop sounds an octave lower.
https://en.wikipedia.org/wiki/Organ_stop#Pitch_and_length

So this 8' stop is one that sounds at the pitch played.

Regale is the name of the stop. It's a standard baroque type of reed stop, which was always popular in Spain. In English it's known as a "Regal":
http://www.organstops.org/r/Regal.html

See also the encyclopedia Taña has cited:
https://books.google.com.jm/books?id=pmRuBwAAQBAJ&pg=PA454#v...

So "el Regale de 8'" is "the 8' Regal":

"The 8' Regal and 4' Gedeckt pipes are inside the body of the rectangular 8' spinet"
https://books.google.es/books?id=LG3DUo0pBckC&pg=PA82&lpg=PA...

"This instrument is designed so that the 2' rank removes ( in one piece) and can be interchanged with the 8' regal (one piece)."
https://www.harpsichord.com/Organs/continuo.html

Finally, "conjunto": this simply means the complete set of pipes corresponding to this stop. Sometimes there is one pipe per note, sometimes more than one. A term you could use here is "register", since "conjunto" is being used here in the sense of "juego":

""REGISTER: Register is another word for the complete set of pipes — one rank or multiple ranks — that makes up one stop."
See this previous question:
https://www.proz.com/kudoz/spanish_to_english/music/6199903-...