Huntsville TX Walls unit "Siren" is a Bell?

[B-FlatMajor]

I was searching for siren locations when I came across this article from Vice.

https://www.vice.com/en/article/ppm75v/ ... d-a-prison

It mentions some sort of system that signals the start and end of a headcount at the Walls Unit. The article refers to them as Sirens but I came across other things that suggest its actually an Alarm Bell. While looking on Google maps, I couldn't see anything near or in the prison that looks to be a Siren. Only a Bell was visible on a roof.

https://goo.gl/maps/vQewxmPR9LDvYrMC6

Could the Bell be the "Siren" Vice is referring to or is there a Siren somewhere else in the area?