Hello @Janineneedscaffeine , I dont know if you succedded in your identification, but I was recently reading these threads which might be helpful :
1st is All shogakukan books with price, date, summary
This a translation from the official Shogakukan website (linked in the post at the beginning) listing all their books, but you will have to buy them if you want to see whats inside.
2nd is Shogakukan Stamps inventory, the author gave a link to his (very clean) website showing a lot of stamps and their year of release.
Hope it will help anyone 