I bought the bike new and assembled. Somehow 'the thing' must have been there all the time and I only noticed it after it must have broken off on the inside of the frame. It looks and feels like hard plastic. Maybe hardened glue or epoxy resin. One side is 100 % flat, the other as can be seen on the picture
Ideas and suggestions are very welcome solving the mystery.
Cheers, Gerard, NL
Next race probably St. Anthony's in St. Pete's