Rambam's Secret Interpretation of "An Eye for an Eye"

Rambam writes in the Guide, 3:41:The punishment meted out to anyone who has done wrong tosomebody else consists in general in his being given exactly the same treatment that he has given to somebody else. If he has injured the latter’s body, he shall be injured in his body… And he who has deprived someone of a member, shall be deprived of a similar member: “As he hath maimed a man, so shall it be rendered unto him” (Leviticus 24:20). You should not engage in cogitation concerning the fact that in such a case we punish by imposing a fine. For at present my purpose is to give reasons for the [biblical] texts and not for the pronouncements of the legal sciences. Withal I have an opinion concerning this provision of legal science, which should only be expressed by word of mouth.

What was the secret opinion Rambam held regarding this law?