Tortuguita is the traditional name in Argentina. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Same general anatomical region (outside-rear-leg muscle group). Tortuguita is a smaller curved muscle within the broader silverside / contra group; both are slow-cook cuts.
Anatomically adjacent — tortuguita is on the upper rear leg, osso buco is the cross-cut shank below it. Both are slow-cook working muscles with significant connective tissue.