Some sites state that Bosko Milenkovic unofficially started Czech Grand Prix in 1949. (Stats F1 and Wikipedia, for instance).
I saw some footage of the race and can't support it, not mentioning that it seems highly unlikely.
Milenkovic was a rich property owner and his belongings were destroyed by bombs during WWII, so in post-war communist Yugoslavia he became poor and started working as a bus driver in Belgrade. In 1955. he hit a boy on a scooter and, believing that boy has died (although he eventually recovered) came back home and hanged himself.
I don't know his T51 serial number but it is highly unlikely that it survived the war and even more unlikely that apparently by then very poor Milenkovic could bring it to Brno and race it. Any Czech members might know more about this strange "unofficial start" or shed more light on 1949. Czech Grand Prix?