Location of 4 conjecture for Markov sails

Let $b/a=[a_1,a_2,\dots,a_n]$ be the continued-fraction expansion of the rational number $b/a$, and let $A_m,B_m$ be the corresponding penultimate-continuant vertices of the Markov sail. Location of 4 conjecture. If $n=2m+1$ is odd, then $\mathcal M(B_m)=4$; if $n=2m$ is even, then $\mathcal M(A_m)=4$. The claim supplies the initial coefficient value used with Markov Sail Duality to determine further sail coefficients; the source gives no resolution.