With the introduction of the 8000 & 9000 offroad groups, Shimano has changed it’s brake pads for mountain and post mount road cooling fin type.
(The image below shows a post mount pad set vs a flat mount pad set)
F03C (Y8J79802A) used to be the metal-type pad used in the calipers. Now, J04C (Y8LW98030) is used. The pads are completely cross compatible although J04C is an improved pad. Slightly raised ‘fins’ have been added to the inside of the pad plate. The shape of the plate and orientation along with the topography of the cooling fins have also been changed, assumedly for improved cooling.
L04C is a special pad for flat-mount road and should not be confused with the others as it is not compatible.
Below, you can see BR-M8000 caliper with J04C pads:
Below, you can see BR-M785 caliper with F03C pads:
Below, you can see BR-M785 caliper with J04C pads
BR-M666, BR-M615, BR-S700, BR-R785, BR-M985, BR-RS785, BR-M785, BR-M675, BR-M9000, BR-M9020, BR-M8000, BR-M987
Another bit of info that most may know. The new post M785 brakes use a shorter SM-BH90 connecting insert, bolt, and cover with SM-BH90-SB hose. The insert is silver in color and the bolt is shorter than the last. Some handy part numbers – Y8JA98010 Connecting Bolt Unit, Y8JA98020 Olive & Connecter Insert, Y8JA07000 Cover