i know of an open/closed cast on..
it is a variation of long tail
One stitch is made 'normally" (a normal long tail stitch)
(a normal long tail stitch is made into a closed loop--one that looks like all those ribbons (pink, yellow, etc)
the second stitch is made into an OPEN LOOP (one that looks like the letter U!)
the result is a edge that is rippled and very stretchy--(i think it is stretchier than the german twist(aka Norwegian, twist, maine) cast on.
here is a link to my youtube on long tail variations.. the open closed cast on is there.
I could be wrong..
ONE real problem is cast ons have so many names.. can you link to a image? i know lots of cast ons.. and i might know this cast on by an alternate name!