Of the various use cases of the Pratt & Whitney JT8D (the most-produced low-bypass turbofan in history, and the second-most-produced turbofan of any description1), those installed on the 737-100 appear to use a set of auxiliary suck-in doors, similar to those so familiar from the earlier JT3D:
In contrast, the JT8Ds installed on other types of aircraft, so far as I can tell, do not have suck-in doors:
(Image by duch.seb at Wikimedia Commons.)
Why did the JT8D need suck-in doors on the 737-100, and not on any of the many other aircraft it was attached to?
1: Behind only the General Electric/SNECMA CFM56 high-bypass turbofan, used on (among many, many other aircraft) the 737-300 through -900.