Vamo's Do have a firmware version.
V3 firmware version can be verified by:
Remove the Battery Cap
Press and Hold the Fire Button
Reinstall the Battery Cap
KSD 2.0 or KSD 3.0 should be displayed on the OLED display
Story is that KSD 3.0 allows switching Mean/RMS (verified on mw Vamo V3) while KSD 2.0 does not (RMS only).
This being true explains why some V3s won't switch Mean/RMS.
http://www.youtube.com/watch?v=FLO3PTUek2o
PBtusardo V5 review shows how to switch Mean/RMS