There is a pair of Beech (American Safety) inertial reels available on eBay.These reels can be readily re-webbed with a longer (up to a point) or different color of strap (re-web references are on BAC). They are typically the most costly part of an aftermarket shoulder belt installation, if the structure is in place for attachments. Some of the late sixties/mid seventies planes had shoulder belts as an option. In the earliest versions, the inertia reel (or plain strap) was just bolted to the inboard floor bracket, along with the inboard end of the seat belt. There was a threaded receptacle near the top of the aft door post, where a flanged pin (having a plastic sleeve) screwed in; I think that the threads were AN6 (3/8-24). The seat belt was fastened, then the shoulder belt was pulled from the reel up across the body, and snapped on the pin above and behind the shoulder. Perhaps some owners of these models can chime in and confirm this description.
In the later versions, and when they became standard (mid to late seventies), the reel is anchored to a bracket in the rear sidewall. The belt exits through a trim cover, is routed up overhead through the D-ring (with another trim cover), down across the body, and snapped on the flanged pin on the seat buckle.
Anyway, I don’t know what the reserve is on these, but in light of recent posts, I thought I’d pass it along in case someone is accumulating parts for a shoulder harness install. If you already have the threaded mount point in the upper doorpost, it shouldn’t take a whole lot to get harnesses installed. If you do it using the Beech reels, and the hardware from the parts books, and since these were a factory option, an informational 337 should be all that is needed (no field approval, no STC). Unless our A&P-IA Guru Bob S opines otherwise!