It should say on the rx case what type it is. If not, you can go to the Futaba or Tower sites and figure it out. You will need the model of the rx to look it up. To the best of my knowledge there is only one type of tx crystal although you need to be using Futaba crystals in Futaba radios. In the US it is not even legal to be swapping tx crystals around, as you are apparently doing, for this very reason.