I'm reading Young Stalin by Simon Sebag Montefiore. The book frequently quotes letters between him and fellow revolutionaries, often when he was in internal exile in some obscure corner of Siberia.
The letters make frequent references to sending money to each other.
How would this work? Would they literally insert a few banknotes into the envelope and risk it going astray somewhere in the intervening 1000s of miles the letter would travel (also, bare in mind that as an exiled revolutionary all his mail would be subject to censorship) or was there some form of travelers cheque or postal order that only had value to the named recipient? He seems to have been sent to some pretty desolate and obscure hell-holes in the middle of nowhere, so my guess is he didn't have access to normal banking services.