As I understand it, there is a text in Pāli, the Pāli 143 verse version of the Anāgatavaṃsa (Pāli)
then there is a development of this text in Myanmar/Burma represented by the book in English, The Coming Buddha Ariya Metteyya.
and a few hundred years later
a development of this root text in Sri Lanka a Sinhalese presentation called the Anāgatavaṃsa desanā in its English translation.
How closely are these related, and if so, how?
The verses are contained in the Myanmar book, And the commentary bulk of each text is different.
Thanks Leo