Synchro ID Question

I have my 2013 Mustangs MT82 apart on my garage floor. The motivation for this was the slow loss of the 5th gear synchro. I now have that crunchy nugget in my hand. When I look up the BR3Z 7107 I find two different parts, an E and an F. The E is the standard sintered surface synchro and the F is a graphite faced synchro. The one in my car is the F version. Can anyone tell me why these were made different and which applications the E version is for. All other synchros I have taken out are sintered - I have not had to remove 6th gear so do not know about it. Thanks for any help.

I can now answer my own question after a bit more research. Have no idea if anyone GAS but in my 2013 version of the MT82 there are two synchro rings with identical numbers BR3Z 7107 and two differing letters E and F. E is sintered metal lined and for reverse, while F is graphite lined and for 5th and 6th. If the price is $25 ish, sintered. $45 ish, graphite - all else equal.