It was an experimental unducted fan engine. They were tested on both 727's and MD-80's back in the late 1980's. They were supposed to drastically cut fuel consumption, but they were very noisy.
In the late 80's I worked in a hangar where they were doing a conversion to 727's, removing the center engine and putting 2 MD-80 engines in on the sides. It was a long process, they had a Danish plane from Sterling in for a long time.
It never really took off, the 727's were on the decline back then, now they are all gone but for cargo.