Leonardo da Vinci wasn't an accordionist (video)
In 15th century there were no accordions, so definitely Leonardo da Vinci wasn't an accordionist, but maybe he had a portative organ (portatif organ, portativ organ, or simply portative, portatif, or portativ) - a small pipe organ. The performer manipulates the bellows with one hand and fingers the keys with the other. The instrument was commonly used in secular music from the 12th to the 16th centuries.
Portativ/Portative/Portativo/Organetto - YouTube