The only difference I know of between the 361 and 361A is the idle jet. Either one will work fine on a TO20. A TE20 had the same exact Continental Z120 engine as the TO20 but the TE20 used a TSX312. I have no clue what the change was for, maybe the 361 wasn"t available when the TE20 was released and was available by the time the TO20 was released. I personally run a TO30 TSX458 on my TO20 and it runs great. Maybe this link might show you any other differences between the two carbs.