The following are three authentic examples from Google Books:
Both women experience loss, from men to dreams, but through it all they remain the truest of friends for 50 amazing years.
Loyalty makes the wolf a good member of the pack, the dog the truest of friends, the savage a good member of the band, the tribe.
We are sharing with a truest of friends and there must also be time to reflect and unburden ourselves, to voice our hopes, disappointments, regrets, and to savor his companionship.
I'm wondering what role the preposition of plays in these examples. Why not just say "truest friends" instead?
Are these two constructions interchangeable? Any contextual nuances implied?