As far as I was aware an Oyster and Freedom card look very different, unless they had their own covers to protect them which were the same. Feasible I guess.
I can't get my head around the whole criminal record thing. If you don't have a ticket, use the wrong one they usually just give you an on the spot fine. Seen that a few times happen to people.
I can only guess that iamjack, when he agreed with the ticket guy he said 'yes i did use it fraudulently' and that is why they then wrote to him and fined him more. They only usually prosecute if they have reason to believe you have done it before or admit to using it fraudulently.
I know someone that didn't pay the fine and they were taken to court, but the getting a criminal record doesn't add up to me in this case if it is as stated by iamjack.
If any of us had picked up the wrong card would we not have said 'omg, i've bought my sister card by mistake, can i pay for my ticket now', not 'i have used the card fraudulently', now please let me go I am nervous and want to leave as soon as possible'.
It doesn't stack for me.
You should not be pursuing this iamjack, they will laugh you out of court because you have already pleaded guilty. If you are genuine, I wish you well.