Key

Original key part number was 776586 as found underneath the the rim of the broken key. It's known as a compactor key, or a key-knob, but in layman's terms, I would call it the "off-on-start" knob. The replacement part number was 386676, and fits perfectly.
The original key had become old and brittle, and eventually just broke apart. This was a 30+ year old Whirlpool Trash Compactor key. I just picked out the bits of the old broken key with needle-nose pliers and then easily inserted the new key into the opening.
Note, this literally works just like a key you would insert into any lock. It doesn't require pushing to "snap" into place, but once inserted and turned to the "ON" position, locks into place. No forcing is required at all.
The new key looks exactly like the original and is working very smoothly.