There is a stamped number series adjacent to the balance wheel (a53/3) on my planet ocean watch as viewed from case back. I think it possibly is the steel grade has anyone viewed this?

 

Steel grade? Not sure what you mean by that...

Here is a photo of a main plate from a 2500C from a PO I serviced a while back...



On the right is the caliber designation, so 2500C, and on the left are a series of numbers and letters. On the right side of the group of numbers and letters on the left "H3" is stamped, and this is the hand height designation of the movement in this watch, so hand height 3. The 2500C comes in 3 different hand heights (H1, H3, and H4), so all parts that are involved in mounting hands to will have different lengths depending on the version, so the seconds wheel, cannon pinion, and hour wheel.

The other letters and numbers on the left side don't really have any significance to a watch owner or watchmaker, and are what I would assume are just manufacturing codes of some kind that track batch numbers or something during the manufacture of the movement.

Hope this helps.

Cheers, Al