https://www.rugbyleagueproject.org/seasons/nswrfl-1964/round-1/parramatta-eels-vs-canterbury-bankstown/summary.html
https://www.youtube.com/watch?v=fVOI5wtXAig
John Cox for Parramatta wore jumper number 5 in this match not 21 as can be seen at 4:14 in this footage.
Thanks Ted.
We generally assume a player played in the number they have been allocated in the program if they were a replacement in the starting team or came on during the game this seems not to always be the case as John Cox was a replacement in starting team for Mike Jackson and was listed in reserve grade in jersey #21 so we assume he played in #21.
It is impossible to know unless there is video of the game.
I totally understand. There isn't much option without video or if there are any club records they would grant access to which is highly unlikely.