It very much depends on which time we talk about. During Stalinism, pressure was most intense (and if we were sure it's a concerto from 1950 I would say a Soviet author should be as good as impossible), while in later years, the variety of styles among Soviet composers is actually large (much larger than commonly assumed), so that I don't really think we can exclude a Soviet composer for reasons of style. To see that a Hindemithian approach was certainly no risk, just keep in mind how many Soviet composers tried dodecaphony (so, even more modernist tendencies) in the 1960s. This said, of course there is also some possibility it's not a work by a Soviet composer, that's correct of course. For instance, I have a Melodiya LP from ca. 1980 with pieces for clarinet and piano by Martinů, Glazachev (Ukraine), Poulenc and Podkovyrov (Belarus), indicating that stuff from abroad was also played (of course).