in the new harvard dictionary, under temperament, it explains that practical compromises are made in desiring acoustical purity (and that for musical transposition or modulation) that is not compatible in a closed system. if the fifths are kept acoustically pure (as in pythagorean tuning), then enharmonic pitch classes such a Ab and G# will differ by an amount known as the pythagorean or ditonic comma, equivalent to 23.5 cents. mozart utilized these positively (as with the violin - playing in a sharp key toward the higher end - and a flat key toward the lower end). BUT, i think Beethoven was looking ahead farther (though he still obviously liked overtones)
in the twentieth century, a temperament with 12 part equal distribution has predominated. in this, each of the pythagorean scale's fifths is diminished by about two cents so as to eliminate over the sum of 12 fifths the accumulation of the nearly 24 cents of the pythagorean comma.
just intonation replaced the pythagorean scale and equally distributed the syntonic comma.
for irregular and enequal distributions (still thought to be more equally tuned - than the well tempered system of bach) was the temperaments of Johann Georg Neidhart (probably read by Beethoven). his model temperament #3 (1732) distributes the pythagorean comma unequally: fifths on Eb and Gb are 1/12 of a comma (two cents) flat; on C and G, 1/6 of a comma (four cents) flat; and on D and A, 1/4 of a comma (six cents) flat. Neidhartdt's own favorite temperament distrubutes the comma unequally to nine fifths in the scale. Those on Bb, A, B, F#, C#, and G# are 1/12 of a comma flat; thos on C, G, and D are 1/6 of a comma flat. Doesn't it seem like Beethoven experimented with those very same keys in the Walstein.