It is from Combat Report and Coastal Command Review 11/1944. It clearly states "... R.P.s were fired in fours salvoes of fours ...".
According to my sources, there were only eight SAP60s on first versions of GR Vs but stow-away double carriers for 16 pcs were later used in the bomb bay.
Feel free to contact me off board.
Michal
fairlop@quick.cz