The position of the Nh8 requires that it reached h8 before h7xg6 took place, so that the missing bl. R has been captured on g8 or h8 and the wPh2 could not reach the g-file via e.g. h4xRg5. Also we would like to say that the promoted unit has been captured on a6. But this needs not be so. Indeed, the promoted unit can very well be one of the wh.
Knights present in the diagram.
The Ceriani-Frolkin theme is even more impressive when it is multiply displayed. Here is an example showing four Ceriani-Frolkin promotions with precise identification of which Pawn is promoted where into which unit and where have the captures been made by whom. History of the wh. Many problems of the Shortest Proof Game genre display Ceriani-Frolkin promotions. Here time-pressure and the necessity of not checking the enemy King are often used to enforce promotion into a specific unit.