Let $M^{ 0}$ be a complete hyperbolic $3$-manifold whose conformal boundary is a closed Riemann surface $S$ of genus $g \geq 2$. If $M=M^{ 0} \cup S$, then let ${ \rm Aut}(S;M)$ be the group of conformal automorphisms of $S$ which extend to hyperbolic isometries of $M^{ 0}$. If the natural homomorphism at fundamental groups, induced by the natural inclusion of $S$ into $M$, is not injective, then it is known that $|{ \rm Aut}(S;M)| \leq 12(g-1)$. If $M$ is a handlebody, then it is also known that the upper bound is attained. In this paper, we consider the case when $M$ is homeomorphic to the connected sum of $g \geq 2$ copies of $D^{ *} \times S^{ 1}$, where $D^{ *}$ denotes the punctured closed unit disc and $S^{ 1}$ the unit circle. In this case, we obtain that: (i) if $g=2$, then $|{ \rm Aut}(S;M)| \leq 12$ and the equality is attained, this happening for ${ \rm Aut}(S;M)$ isomorphic to the dihedral group of order $12$, and (ii) if $g \geq 3$, then $|{ \rm Aut}(S;M)|<12(g-1)$, in particular, the above upper bound is not attained.
Categories: math.GT