The publication by S.V. Levin and V.A. Solomonov contains correspondence between N.M. and E.I. Druzhinins and a Saratov historian I.S. Porokh in 1959-1986. The letters are kept in the Archive of the Russian Academy of Sciences and V.A. Solomonov’s personal archive.
LEVIN et al. (Wed,) studied this question.