91A was the official Hospital Medic
91B was the Combat Medic.
Then later one they changed 91A to 91C when they gave them the LPN 91B stayed the same.
Then the army decided to combine the two (Most likely to make it more apprealling for recruiting) and they called it 91W. "healthcare specialist" if you happen to get LPN school you were known as a 91WM6. Everyone else got EMT-B. you might call them "Combat medics". (I was the frist class that they converted to 91W)
3 years ago, the army removed alot of classification of jobs they no longer had. so 91W was turn into 68W.
Hope that answer the Question