There´s a D major chord there, so the B is not in context. We cannot even consider it as an added sixth, as Respighi, who was a great orchestrator, would have doubled it somewhere else. The same with the E, which we cannot consider a ninth, as no other instrument is playing that note. It is a normal, plain, D major chord. Even more, do we really think that we could get a substantial B and E from a wooden plank so it would be the only instrument playing those pitches in that harmonic context? That makes no sense at all.
So, IF EVERY SINGLE NON PITCHED INSTRUMENT is written using a clef, why is it that we have only worried about assigning pitches to the “tavolette”? It makes no sense at all; we should assign pitches to all of them or to none. What is the solution? The obvious one; indefinite pitch instruments make noises, they don´t play pitches. “Tavolette” are one of them, and if we have not worried about assigning pitches to the other percussion instruments in the score, worrying about assinging them to the “tavolette” is absurd. “Tavolette”, noise making popular instruments. Do not over think it, as it is quite clear that, knowing the “matraca”, they are non pitched instruments. Even more; if finding “tavolette” is quite hard in non-Mediterranean cultures, how hard do you think is finding tuned ones? Hard, very hard. So hard that they do not even exist, as they are non pitched instruments! Tuned “tavolette” are as rare as a green unicorn. Do not search for them, as you won´t find them (both “tavolette” and unicorns 😉).
So, the next time you play this piece, use the same common sense with the “tavolette” part that you use when approaching the bassdrum/cymbals part, the triangle one or whatever non pitched instrument part featuring clefs and pitches; realizing that it is an edition issue.
Having said that, playing this piece is a wonderful oportunity to experiment trying to get a beautiful, musical and popular “tavolette” sound (and, obviously, enjoy a fantastic piece of the repertoire).
Please check this old recording of the RAI Orchestra. You will see that the supposed intonation of the tavolette does not appear anywhere: