One of the reasons you likely aren't seeing a lot of answers here is because the use of the phrase "beyond a shadow of a doubt". While many inventions were attributed to Thomas Edison there is considerable controversy about other influence and input into that work. His earliest inventions took place while he was employed with the Western Union Telegraph company and it is very probable that anything invented there was not a solo venture. The best case can probably be made for his improvements to the stock ticker devices popular at the time. It is one of his earliest inventions having been patented when he was only 22 years old and sold to Gold and Stock Telegraph Company for $44,000. It was with this money that he began hiring staff and working more as a leader of a group of inventors rather than a solo inventor himself.