Time has passed, but I thought I could help. I seemed to remember that the sentence "poner el dedo en la llaga" comes from the Bible, and a couple of web sites seem to say the same.
Looks like I can't post the URL, so I'll have to decompose it a bit...
www .1de3. com/2009/05/05/poner-el-dedo-en-la-llaga/
So, looks like the saying comes from Thomas, who wanted to put his finger into Christ's wounds to believe that He was resurrected. John 20:25.
However, the meaning clearly isn't the same... Strange.