Photos don't show the faceting in detail but I would venture to guess the diamond is a Modern Round Brilliant, possibly an early MRB. Nothing in the photos show anything that would lead me to think it is a transitional or an antique-cut diamond.
Wow! Thanks for all the great info!! I am amazed at the knowledge you guys have!! And lightbright your ring is gorgeous!I think early round brilliant too. Large table with lower crown. Cool vintage ring!