The Steel Heart II was about 1-1/2" longer in the blade than the Original Steel Heart, which became known retroactively as "Steel Heart I" after the Steel Heart II was intorduced. The Steel Heart I was ATS34 stainless, and the Steel Heart II was A2 steel. There were no other differences, both had the "bird's beak" type pommel and front and rear talons with talon holes.
After the Straight-Handled Steel Heart replaced the Steel Heart II, it gradually also began to be referred to as Steel Heart II by the then newbies. Today if someone refers to a Steel Heart II, it is best to inquire whether they are talking about the Bird's Beak Steel Heart II or the Straight Handled Steel Heart II to avoid confusion.
INFI steel was first used for the Straight Handled Steel Heart, although I think that the very first ones were of A2.