I wonder how during WW2, ATA pilots managed to convert quite well to new, and often complex, types with no check flights and just a set of universal pilot notes. And how does one get checked out to fly single seaters? A good briefing and read the flight manual. Seems to work OK.
What was the average number of flights a pilot conducted before getting killed? Don't need much training for that. Even later, i.e. Korean war, warbirds were designed for a one-digit number of flights ... but this is getting off topic now.