Nice round up Leo…ehhh, but I'm nososure about your v4 thing. These smaller horiz feed (smaller air hole) immediately followed the un-numbered change to the orig RBA (cross-cut) slot. Really the v2's. They've been around for a while. I like these for the (closer to) inline wick positioning and wish they were 3mm. They came out after all the raggin about inadequate juice flow. If they had been 3 or 3.2 mm like the subsequent variations with lower holes, that would've been perfect for most styles of wicking (to deck or straight-thru).
Vin's published pictured slot drill-outs early on were great. A bit of a PIA but helped a lot. I drill out both slot and a matching arc or half-moon into the rba's tank wall where it meets the slot when threaded. Downside is that the threaded lip is thin and the tank's threading actually spreads with use so slot and wall cuts don't line up. So you have to do some
guesstimation on a fresh one and hopefully you're lucky. When you are, it's swell.
Right on re the qual of the originals. I have a few you pictured (pair) with air holes apparently bored to the max. A Dremel's worth it to get there otherwise. You can bore these out sideways into ovals as well to help max out the airflow. Very useful if you wick chunky.
Thanks for the memories Leo.
Good luck.