The original marking "Carl Zeiss Jena Tessar 2,8/50 5369512" on the lens was masked with a ring "Jena T 1:2,8 f=50 Nr.5988069".
Both serial numbers date this lens between 1958 and 1961 which corresponds very nicely with the 1958 Pentacon FBM on which I found it. Could it be the original lens with which the camera was sold?