I stand to be corrected here, but I think it had something to do with converting DC3s and C47s back to Australian civilian use after WW2.
The DC3 first operated in Australia in 1937 or 38 and it was similar, but not identical, to the military C47. The DC3 was originally powered by a pair of single row Wright 1820 Cyclones. I think they were about 1000 HP.
Airline service was interrupted by the war when the US military impressed civilian DC3s into service and took over the production run, with C47 and other military variants, of the type. Compared to the DC3 the C47 had a stronger floor, different passenger door, a large parachute door and was powered by P&W 1830s of 1200HP.
When the war ended military DC3s and variants were returned to civilian use. In Australia a bunch of them were bought by ANA and TAA and they were all converted to a common engine configuration - P&W R1830s with fixed cowls.
Thats how I remember it being explained to me, in the 80s, by an Ancient Pelican who knew this sort of stuff. There are are a few in D&G who will correct me if needed
Cheers!
Pithblot