Ford: oil filter

Question
I have noticed the fl-1 has different letters after
the 1 what is difference

Answer
Which letters are you referring to ? The A ? As opposed to the S after all the new filters ? The A is just a generic number for sticking purposes . The S on all the new filters lets you know it has the check valve installed which holds oil if say it's installed at an angle or upside down the prevent cold start lubricatiom loss