The Watchtower has held for many years that Jesus died on a stake and held strongly to this teaching.
W 1951 3/15 The above agrees with the New World Translation of the Christian Greek Scriptures in its Appendix, page 769, in saying that the instrument upon which Jesus was nailed was a stake without a crossbeam, and not the religiously represented “cross”; and that the Greek word used for that instrument in ancient time meant a “stake” and not the conventional religious crossThey are no longer certain on this point. This is from their website on the question of whether Jesus died on a cross or stake
Many view the cross as the most common symbol of Christianity. However, the Bible does not describe the instrument of Jesus’ death, so no one can know its shape with absolute certainty.
This is an issue that was strongly upheld when going out in field work. So many arguments would have been prevented if we could have said that there is no absolute certainty on the matter.