I think a 6X5 (Type 4) in black or plum would be fine. A Bakelite 6X4 (Type 3) would be correct too. A plum 6X5 would look the coolest, but they're expensive ($140 right now). Those are Russian options, however. Bulgaria only made black 6X5s...that I've seen. They also did some nice 6X4s that look just like the Izhmashs and Tulas of the period.
Interestingly, I've noticed that the Bulgarian Bakelites come in a dark and a light variation. The shade differences between the two are pretty significant. Sometimes the two versions get combined (e.g., light scabbard with dark hilt and vice versa).