I'm working on a research paper about Bitcoin, and would like to give a quick historical introduction into how Bitcoin evolved. I'm obviously aware of HashCash as a precursor to the PoW-aspect. Is there also an earlier system that used digital signatures to transfer ownership of "coins"? My impression so far was that Bitcoin "only" innovated in solving the double-spending problem. But looking around, I can't really find any earlier system where such a signature scheme was proposed.
I can find Hal Finney's RPOW proposal (although information is not really dense about it), but from what I found, this is more like Chaumian cash in that some central instance is used to issue and redeem tokens. I appreciate any pointers to earlier ideas that proposed already the signature scheme - if there are any. Maybe my feeling is just wrong.