OK, friends, I'm standing corrected:
I'm at home now, with the story in front of me. The relevant detail is buried in one of Annie's long run-on sentences (which is why I'm not typing out the whole sentence):
...the submarine Thresher lost two months earlier with all hands and how it must have been in the last doomed minutes...
Like I said, a memory like Swiss cheese.
In the story the conversation that included the
Thresher apparently took place the night Jack noticed Ennis washing, after they had already switched jobs.
I'm sure there must have been a discussion somewhere about when the sheep would have been taken up on the mountain, but searching isn't my strong point.