Hi hope someone can help please.

I am currently rebuilding my son’s NSR250 MC18 R5K engine and I am trying to identify which Cylinder is the front one.
The one cylinder has F (No. 13121837) and the other has F2 (13121838) stamped on the Barrels.

Both cylinders will need Niksil plating and new Pistons which I am in the process of sorting.

Many thanks
Philip