A back to die for

This cover is quite nice on first glance, though nothing special:

Registered, double-weight letter from Blagoveshchensk to Zuerich

It's a perfect respectable 50k rate letter from Blagoveshchensk. These frankings puzzled me for a bit since you normally see 40k frankings (20k postage plus 20k registration), but it appears that 20k was postage for the first 15 grams, with each additional 15 grams costing 10k, so 20k+10k postage and 20k registration. August 1922, so sent during the last few months of the FER, although the Chita stamps remained in use until February 1924.

But the back makes this cover a rarity:

Charity labels! In Gold kopeks! From the Ts.D.V.K.! If only I knew what that meant! (I'm betting Central Far Eastern Committee)

Only cover from the FER with charity labels I've seen.