= Solution
There are \b[two carbon-placement isomers], represented by the cyclic base sequences $\mathrm{CCBBB}$ and $\mathrm{CBCBB}$. The <carbon> vertices are adjacent in the first and separated by one <boron> vertex in the second. Rotation or reversal of a sequence does not produce another constitutional isomer.
Each arrangement has a <reflection symmetry> that exchanges a pair of <boron> sites while leaving the third distinct. Consequently each can give \b[two $^{11}\mathrm B$ environments, with site populations $2:1$], in a <proton-decoupled NMR> spectrum. This assumes that the <hydrogen> and <carbonyl> arrangements preserve, or rapidly average to, the corresponding skeletal symmetry. Two peaks alone neither choose between the two skeletons nor establish a unique <hydrogen> arrangement; accidental overlap or <fluxionality> can also reduce the number of observed signals.
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