I'm suspicious of how common these marks are without any more impressive reference to modern scholarly literature. Allen, for example, says the following:
"About the end of the republic a new device makes its appearance -- the so-called 'apex' placed above the vowel symbol; this, however, does not appear on the vowel i until the second century A.D. The shape of the wymbol varies, but a mark like the acute accent (') is characteristic of the empire, and [two symbols that look like a smooth breathing that I cannot produce on a QWERTY keyboard] of the republic.
"Apart from such indications, which are not infallible and only sporadically citable, our knowledge of vowel length in Latin comes from various sources..."
Most men are rather stupid, and most of those who are not stupid are, consequently, rather vain.