Philatelic covers are reasonably common but examples of genuine postal use are very rare. Alexander Epstein has tried to make an Inventory of known examples. A few years ago, I was able to add one to his list.
Now I think I can add a second:
Good day, Patsits Diu ... Fedorovich. I want to let you know that I am alive and well and that I hope that all is well with you. Dear Brother, I am now alone. My family remained in Petrograd. I don't know if you are now in Petrograd or already somewhere else. Can you please find out some news about my family. Then please write to me at my address: Town of Gdov, Petrograd Prospect, Customs [?] D... Bojarov, T.O.Kremen"- after which he concludes with the passage I have already quoted.
What happened next? On the reverse of the card is another postmark: WARSAWA VIIIb, 15 V 21.