In the attached image I have a scenario where very similar moves are illegal. I'm pretty sure the ONLY reason the move on the right had side is illegal is the no previous board position rule, however I want to make sure I'm not missing anything about the mechanics. Is there any other reason this move is disallowed?

Without context, I have no idea why you'd claim the second position is illegal. Looks legal to me.

_________________
- Brady
Want to see videos of low-dan mistakes and what to learn from them? Brady's Blunders

Could you also post the entire SGF up to that point ?

It's obviously a ko

Hi,
I don't see the move of the first diagram. The cross is on the S11 stone, but the R12 stone looks like it was just captured.

Except that playing S11 doesn't capture R12 !

The first diagram on the left is black to play as indicated by the shadowy black stone. The second image is white attempting to retake this ko as indicated by the shadowy white stone.

S11 was the last move before that one (no capture)

The diagram depicts black to play about to capture Q12.

The second diagram depicts that move having been played and Q12 now gone. White is to play and I was attempting to place at Q12 to capture R12. I wanted to make sure the reason was ko and not some other rule I don't understand.

Ok, I misunderstood your order of moves. I think I understand now. This is a common situation called a ko, as explained here. Hope it helps!

_________________
- Brady
Want to see videos of low-dan mistakes and what to learn from them? Brady's Blunders

Aaaaah, ok, I understand now.

Yes the move is forbidden because it is a ko. According to the rules used, the formulation is a bit different.

Japanese & korean rules : two stones that recapture each other alternatively is called a ko. It is forbidden to recapture in a ko.
Chinese, AGA, NZ, French : it is forbidden to create a repetition during the game (repetition of position in the chinese rule, repetition of position + player to play in the AGA rule, repetition of position by the same player in the french rule).

The british rule forbids both to recapture a ko and to recreate a position already created by oneself.