I don't imagine pullovers were supplied with a boot lace threaded through the neck, so this must be a modification?
Is it an "official" modification? Were particular units prone to do this? Was it something done for cold climates to ensure as little heat as possible was lost? Or was it done if the soldier's neck was thinner than the rest of his body would suggest? Or was it done if the wool got rather stretched, although this one seems to have had little or no use? This is not a quiz, I just wondered what it was all about.