U-339 (11/16" hole) was made by Pittsburg in the late 1910's. The Fred Locke large loop (1-5/8" hole) is U-339A. The smaller Fred Locke loop (3/4" hole) is U-339B. Fred Locke's loop insulator design was not something experimental -- he made two styles for large and small conductors. Then Pittsburg made a similar style about 15 years later. Even later Locke (catalog 1941) produced U-339C.

The first three styles were probably made to carry heavy conductors securely in a substation. The later Locke style found with a brown glaze served as a wireholder by Baltimore Gas and Electric Co. on series street light circuits and on low voltage runs along poles at test transformer locations (see CJ March 2007). A white glazed U-339C was found in a bin in the electrical department of Bethlehem Steel in Baltimore.