Lots and lots of pictures of H-19 helicopters. I am using the internet to search various pictures of aircraft plus have several interested parties who worked on these aircraft during their professional careers providing images as well. As with any aircraft that saw an extended service life there are many minor variations, usually in the cockpit and cargo bay, so I try to pick the most commonly occurring large details and keep them in the picture. I'll never claim that these sets are 100% accurate but I can say they are a heck of a lot closer than what Revell did in 1955.
One example is the sound deadening material in the cargo bay, I'm not going to replicate this but will keep the basic seats and bulkhead details. Once painted and enclosed inside the impression should be good and if the modeler cares to add material then more power to him. I will have at least provided a good, workable foundation.