Well, journey from Russia to North Korea by train is probably one of the world trickiest rail journeys you can get
We don't have any experience buying such tickets, however have a few comments and suggestions, which hopefully can help you a bit
I first contacted DB in the UK who can access and sell tickets across most of Europe. They were able to find both Khabarovsk and Tumangan in their system but it seems they could not find journey timings for non-mainline services (they were able to find these for the Trans-Siberian and Moscow to St Petersburg for example) or perhaps could not find journey timings for international services?
It's almost useless to make tries to purchase MPT tickets (Russia to China, N.Korea etc.) in Europe, since DB, CD, SBB etc. all sell tickets using East-West tariff, it "ends" with Russian border, so pretty sure that it wasn't even Tumangan as station, but just Tumangan (gr.), which is a border point. As result even theoretically they won't be able to sell you neither ticket, nor sleeper reservation to North Korea. Non railways company affiliated ticket agencies in Europe, unless they have some kind of connections in Russia (=thus just serve as unnecessary "middle man"), won't help with such tickets as well.
Anyway I also contacted Real Russia a Russian travel agency specialising in rail tickets.
Would suggest to forget about these guys, since it's nothing else than a rip-off agency specializing on reselling Russian train tickets to the inexperienced foreign travelers, obviously while charging enormous commission
MZHA (Moscow Railway Agency)
This will be your solution, imho. However MZA, from what we know, is a bit old fashion place, so doubt you'll be able to arrange tickets just via email. Actual visit to the ticket counter in Moscow might be the only option here.
Now about our suggestions... Since you already found pics of the tickets to North Korea you probably aware about well-known hero rail traveler Helmut and his
famous journey? So we suggest you to contact Helmut (there are contacts on the blog page + he is frequent poster on some European rail forums). First of all he is way more experience particularly in terms of journey to the North Korea. Second, he had (and hopefully still has) some contacts in MZA (it was them who arranged all tickets for him), don't think he would mind sharing his contacts. All that might really simplify your task.
p.s. also we would like to remind you that according to Helmut (thus around 2008), travelers shouldn't really count on getting to North Korea by train from Russia, because it's a risky task (not in terms of safety, but just North Korean border/international tourism politics) and that him and his friend were just really lucky that they actually managed to cross the border to North Korea via Khasan-Tumangan