SF Cable Cars, I have a question...

Good morning:
Although I'm quite familiar with the cable car technology, I have no clue what that slot (marked in the picture) is for:

Maybe someone can help me solving this mystery.
Thanxx.

Have you asked the Municipal Railway Engineering Department?

From the webpage

Questions? Our cable car grip operators and conductors are there to help. You can also Tweet us @sfmta_muni. Or contact the San Francisco Customer Service Center, dial 311 (outside San Francisco, call 415.701.2311).