I don't follow you. What would you like me to understand by "Without grasping the difference involved herein, I couldn't have replied"?
Are you trying to say that you didn't understand my explanation and thanking me for nothing? Or are you trying to say that I helped you to grasp the difference?
Sir, you helped me to grasp the difference. Thanks.
PS: However, the intervening period has given rise to a further question which I will be posing soon.