Ok, hope this is not a stupid question. Are these two different animals? Duckbill earlier? If so when was the change?
Thanks,
John

Yes, they are different.

The "duckbill" was used on the Colt 601.

The "3-prong" is a strengthened version of the duckbill, which apparently wasn't very durable. The prongs have a triangular cross-section instead of being straight. That's the best way I can describe it.

Take a look at Ekie's M16 variation thread, he has put a lot of time and effort into it. It should answer your question and then some.

www.ar15.com/forums/topic.html?b=3&f=118&t=241681

Thank you Sir!

I guess that's why my USGI 3-prong wrenches needed some mods - probably made for the duck bill FH.