I have seen one, and only one, M-64 with 4" tapered barrel with round butt. That was at a gunshow in FL about 15 years ago. I was too poor at the time to buy it, or I would have.

So they did make them, but that was the only one I've seen. If I see another, and have the cash, I'm buying it.

I definately want to get a blue 4" tapered RB, since I carried one in the Army for several years.