Today I discovered the difference between the Hubbell EBK and CM2 all black 5-15P and 5-15C Edison plugs. The strain relief jaws on the CM2 devices barely fit 12/3 SOOW, the EBK strain relief inlet is almost twice as large with will fit much bigger cable, likely up to 10/3 SOOW.
Hubbell tech support did not mention this difference when I called, so I'm posting here for reference and to save someone the hassle I just had.
HBL5266CM2 and HBL5269CM2 are the small ones HBL5266EBK and HBL5269EBK are the big ones. I don't know if other devices are affected.