I have a 2004 FLSTCI. On the OEM mufflers there is an epa warning. FXS 1450. I understand there is also FDX 1450. What do these mean?

 

I don't think that is and EPA warning, but rather a fitment. 1450cc = 88ci, and FXD/FXS being models. I am assuming you meant FXD and not FDX.

I could be completely off base though