The minim tremolo was added – certainly by Chopin – in the stage of proofreading FE1. We replace the "tim" abbreviation used there with the contemporary "Timp." The musical notation is of a conventional nature – Chopin must have meant an octave tremolando 
. This is not an accurate reproduction of the timpani part either, since in the timpani the tremolo lasts only a crotchet and ends with a single note on the 4th beat of the bar (quaver and rest). The remaining sources of the piano part contain here only the solo voice rest. See also bar 260. The described addition was also introduced by EE3, undoubtedly on the basis of a comparison with FE.
Compare the passage in the sources »
category imprint: Differences between sources
issues: Authentic corrections of FE
notation: Pitch