https://www.rugbyleagueproject.org/competitions/craven-mild-cup-1978/game-1/balmain-vs-canterbury/summary.html
Very limited information on games here, but here's what I can add for this one using newspapers & The Canterbury Annual Report.
Round 1 Canterbury V Balmain Saturday February 25 1978 (8pm)
Canterbury 15 (P.Mortimer 2, Brentnall tries, M.Hughes 3 goals) d Balmain 10 (Wigham, Knight tries, Cook, Fish goals) at Leichhardt Oval. Scrums: Canterbury 10-5. Penalties: Balmain 10-7. Crowd: 4.574. Referee – Gary Cook.
Teams
Canterbury: 1. Greg Brentnall, 2. Peter Mortimer, 3. Mick Ryan, 4. Mark Hughes, 5. Greg Mullane, 6. Tim Pickup, 7. Steve Mortimer, 8. Graeme Hughes, 9. Peter Cassilles, 10. Gary Stevens, 11. Bill Noonan, 12. George Peponis (c), 13. John Coveney. Repl. 14. Geoff Robinson
Balmain: 1. Neil McDowell, 2. Larry Corowa, 3. Dennis Bendall, 4. Wayne Wigham, 5. Mike Fish, 6. Percy Knight, 7. Mick Schell, 8. Neil Pringle, 9. David Grant, 10. Dave Edwards, 11. Geoff Naylor, 12. Noel Maybury (c), 13. Keith Cook.
I can confirm all the above players played in the game except for Edwards (Bal). There may have been replacements for Balmain, but I couldn't find any evidence. Stan Cutler was a replacement in either this game or the round 2 game against Newtown. My hunch is it was against Newtown.
I have noted that Stan Cutler was a replacement in this game but I can't find the reference now, this is what I have loaded in RLP.