Following the changes with Revell USA over the last few years, Atlantis Model Co. has acquired molds of classic Revell and Monogram kits. Some have not been released in quite a long time.
The Allison 501 D13 (civilian), or T56 (military) engine entered service in 1954 and is still in production under the Rolls Royce name and designated T56. It was originally developed to power the C-130 Hercules and has since been used in aircraft like the P3-C Orion, Electra, E2C Hawkeye and many others.
I understand Atlantis worked to improve the molds. I could see that the molds in this kit had either held up really well, or had some work done to improve and clean them up.