BigMac *is* SFE 1.x, HappyMeal *is* SFE 2.x. Thus my disparagements re SFE 2.x are rubbish.
However, the comments about SFE 1.x, are correct, as are the note that neither was "officially" supported in sun4c.
I have also confirmed that SFE 1.x is not capable of full-duplex at all: http://www.ipv4security.com/solaris/Fast_Ethernet_PSD.txt (a copy of the original Sun Product Support Document for Fast Ethernet products).
Regards,
Mike Spooner