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:
The always-useful 737 Technical Site confirms that these are, indeed, suck-in doors:
The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.
Auxiliary inlet doors were fitted to early JT8D's [sic] around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.
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.