Holes are for securing safety chain and cotter pin
A nut, flat on top and bottom having 6 flat sides. For items which have internal tapered or straight (parallel) pipe threads, specific electrical conduit size designation, and which when installed, provide a method of tightening to lock the connection, see locknut, electrical conduit. See also locknut, tube fitting. For items having a thickness (or height) exceeding two times the width across the flats, use post, electrical-mechanical equipment. (when height or thickness and/or width across the flats is designated as a tolerance dimension, the maximum dimension will be used to determine compliance.)
NSN 5905-01-498-5227
NSN 5960-01-362-3291
NSN 6150-00-233-8035
NSN 5895-01-061-7634
NSN 6645-00-983-2046
NSN 6150-01-248-5822
NSN 6685-01-272-3867
NSN 7035-00-173-2281
NSN 5835-00-550-7728
NSN 5996-00-548-5179
NSN 6320-01-379-1182
NSN 6110-01-328-5258