TermExplanationExamples
2I-N1 2-mestnyj Istrebitel, Napier-Lion 1 dvigatel, Two-seat fighter with one Napier-Lion engine
Such designations appear until mid-30s, but are not common.
2I-N1
2U-B3 2-mestnyj Uchebnij, BMW-3 dvigatel, Two-seat trainer with BMW-III engine; 2U-B3