Raiden questions

[SynthRicardo]

Does anyone know the difference between (set 2) and (set 3) as they're called on MAME?
What about how the P icon works exactly? It seems to be based on time survived with perhaps some luck beyond your first death (i.e. if you die too quickly it doesn't appear, but if you survive long enough it will appear when dying the first time; and after that it's not consistent afaik; though it can show up as a normal power up dropped by the corresponding enemies). Is it absent in (set 1) and was it added for the other sets? I'm used to playing the second one.
Also, what is the real rank difference between player 1 and 2?

[qmish]

*Set 1 is initial release with several bugs and mediocre reception. The game was called kusoge at launch.
*Set 2 had various bug fixes and is the version where P items work properly for the first time.
*Set 3 had various bug fixes and a toggle enabler to show either Seibu or Tecmo in the attract reel.
*P Item works in the way that a Fairy will release a P item if you die at full power. P items are released randomly on later lives if you survive a while at full power. Extremely rarely it can appear as a pickup from a normal drop.
*The difference in rank between 1P and 2P is one point, because the game mistakenly treats 2P side as -1 rank as though you had just died at the start of the game thanks to archaic programming oversights. In the long run, rank will cap out 1 rank lower with 2P side than 1P side.

Thanks, Emuser. Reposted here so it will not be lost among many posts.