Before the Federal Stock Number (FSN) system (1950s–60s), the Army-Navy used “MPD” for Miscellaneous Parts Directive . “055-28” might be a specification sheet, and “OL” an ordering lot. If your part is from a WWII or Korean War-era vehicle, this fits.
To summarize, most likely refers to an aerospace or military fastener/fitting defined by a Master Parts Drawing. The suffix OL indicates special length or coating. It is not a current mass-market part number. MPD-055 -28 -OL
: This often designates an "Open Type" or "Outside Link" configuration. Unlike fully enclosed tubes, this version allows engineers to easily snap the links open to add or replace cables without having to disconnect the entire machine—a massive time-saver for maintenance crews. Practical Applications You will find the MPD-055 -28 -OL working in: Before the Federal Stock Number (FSN) system (1950s–60s),