As well as UNC and UNF the Unified thread system also specifies UNS (special) threads for use where the other sizes are not acceptable. The finest standard pitch for a 3/4" UNS is 27 tpi. If your thread is definitely 28 tpi then it could be from the Unified constant pitch series. These threads, specified as UN, are based on pitches 4, 6, 8, 12, 16, 20, 28, and 32 tpi and apply to a range of diameters, including 3/4". These are used in applications where the design considerations exclude the other series.
By the 1950's the Unified series fasteners were in almost exclusive use in the motor and other industries, so a probable match for your thread would be 3/4"-28 UN. As for all Unified threads the flank angle is 60°.