so I picked up this valve and had another hanging around. I know they are between level 5 and 7. Are there certain things that set the 6 and 6.5 apart from the 5 and 7. I'd like to get an accurate description to be able to have it noted and hopefully have a nice article written by bunny that is a bit more in depth.

How many valves were made as level 5's?
How many have not be upgraded?
When was the level 5 valve released?
When did the upgrades to the threaded power tube begin?