Consider the vac diaphragm ones as the Mk1. Stopped production in late 2017 or early 2018.
They are distinguished by a sliver coloured cap jewel.
The problems with these are that a number had problems with hoods that split, or the diaphragm didn't work well for some reason. The diaphragm not working was caused by either it not seating properly, being torn during assembly by Wing Sung or tearing in use. The plating on the sliver jewel was poor and wore to grey very quickly.
The Mk2 uses a piston. Production throughout 2018.
They are distinguished by a white coloured plastic cap jewel.
The piston uses most of the same mechanical components as the diaphragm ones, replacing the diaphragm with a double seal piston. I cannot detect a difference in the way the Mk1 & Mk2 work in use.
The Mk3 uses a piston. Production seems to have started in 2019 (ish).
They are distinguished by being double jeweled, with blue jewels.
Otherwise they appear to be identical to the Mk2.
I have bought 7 so far.
2 were Mk1. The black one's hood split. The red one works still.
4 were Mk 2. The black one's hood split. The transparent green, green and flashing blue all work perfectly.
1 is Mk 3. The Red works perfectly now I have put the Mk2 black's cap on it. I think the Mk3 cap sealed too well and sucked ink out on uncapping.
I want a clear and a black that works, but I think I'll wait until the Mk3 comes in black.
Hope this helps.