Given this fragmentation, I have chosen a fairly recognized figure in the Isshinryu world, the grandmaster: Shimabuku-soke. He performs Seisan in this video. I am curious about the opening move (so, 0:08 to 0:012 or so). I don't want to ask a silly question like "What is the authoritative bunkai" -- if there were such a thing, we'd all know it, I'm sure -- but what is a -reasonable- bunkai for this small sequence? You are welcome to extend the bunkai to the next move if you feel that the section I have chosen is too small.
There are many things which factor in this question. The fact that the first step is forward, for instance, as well as the spirit of the form itself.