Hi fgsltw, look up a site called .org, and you'll see a list of all his poems, searchable. For this poem, you'll need to look at the 3rd verse. What you might have to do is to just pencil a few possibilities for the entries where you think there have been changes so far. Pretty soon you'll get a hint as Wintonian suggested. I went a little differently, thinking that the letters from 3-7 down spelled 'thing'.
Then it was simply a question of searching the poem for that string of letters.