The builder is: TVM (Tartu Valumehaanika Tehas), Tartu, Estonia (now Valumehaanika AS) (Thanks to Vuori Susi for the clarification)
ГОСТ 8591-57 (standard) Л - лёгкий (light, as type of a cover according to ГОСТ 8591-57) ГТС - Городская телефонная сеть (City telephone net) 5т - (load for light type) TVM trade mark (Thanks to Vuori Susi for the clarifications)