Not sure what you are talking about. A drop checker looks like:
Safer to add a check valve between the regulator and the tank. There have been reports of check valves between the bubble counter and needle valve failing.You dont really need another check valve. The bubble counter you have likely has a built in check valve so nothing will siphon backwards past the bubble counter. Water siphoning into the bubble counter is no big deal. It needs water if you want to use it as a true bubble counter.