Ron's knowledge of the Pettigrew-Sirius encounter in 1981

[hi_t22000]

No: HPFGUIDX 93504

Referring to PoA (pages 159-160, Bloomsbury Paperback), Ron tells
Harry that his father told him that Pettigrew's mother received only
Peter's finger along with the Order of Merlin after his fatal
encounter with Sirius. 

How did Ron know this? According to Ch 11 in PoA, Ron said this to
Harry on the morning after the Trio overheard Mcgonagall & co in
Hogsmeade. The matter of Pettigrew's finger didn't come up then. And
Ron did not possibly talk to his father that night. And if he had
known this from before, was Ron hiding from Harry the story of who
caused the murder of the Potters?