Well, it should be an ID for library entry, it only known afterwards (like when the season ends), until then it's something like <api_id>_<season_name>_1.png and <api_id>_<season_name>_2.png (then renamed by Ckwng when library IDs are known).
I was already uploading this stuff, I guess I will try to use those placeholder IDs then :v
..Try sortie with mltiple unused (brand new) fighter planes of same type at places where there are no enemy air power and then you can see they gain rank at different speed which make it hard to tabulate the levelling speed of planes. Even more, in this scenario some plane on smaller slot can gain rank faster than planes from larger slot. Thus I think it isn't really possible to tabulates the levelling speed.
Welp... it's a E7 event, and I bet we're both not so prepared for it...
but for tha race, let's go up to E3 Hard and the rest u can pick up Medium or Hard as you wish, how does that sounds? (btw I sent this here 'cause that's a way of helping me not forgeting about it too :v) well... we're still very far from it, but I wish you good luck