Please forgive my not being fully correct in claiming that all tubular back-sights had V sights underneath. This is really only true up to the mid 16th century.
From ca. 1550 to ca. 1600, there were tubular back-sights, of both iron and brass, that were dove tailed at the end of the barrel, in the direction of the muzzle, with no V sights at all. As they detached so easily, they are often missing with only the dove tail remaining.