GR translations: except for William the very first

[Mike Jing]

P27.20-28   Constant saw, and not only with his heart, that stone hand
pointing out of the secular clouds, pointing directly at him, its
edges traced in unbearable light, above the whispering of his river
and slopes of his long blue Berkshires, as would his son Variable
Slothrop, indeed all of the Slothrop blood one way or another, the
nine or ten generations tumbling back, branching inward: every one,
except for William the very first, lying under fallen leaves, mint and
purple loosestrife, chilly elm and willow shadows over the swamp-edge
graveyard in a long gradient of rot, leaching, assimilation with the
earth, . . .

Does it mean that everyone in the Slothrop family saw the hand in the
cloud (when they died, I assume), except for William Slothrop the very
first?  Why the exception?