You stated :
"They used it for shaving with the razor blades they kept sharp by putting them under pyramids."
This is a rather fine exposition of Ancient Egyptian temple art. Lots of study behind that statement.
In case you somehow missed it, what we're attempting to do here, is ascertain just what the "bulb" is. Not what it is not. So far the candidates are :
1. A plasma tube
2. A bubble
3. A sarcophagus lid
4. A representation of the Sun God
It would be reasonable to assume that the Ancient Egyptians had something specific in mind when they carved that contentious relief.
Do you wish to include your learned opinion in this list, or would you prefer a second go? Or do you just want to sit this one out?