Viewing glass

[TEN613RPro]

New trdp and looking at the cover over the radiator area, can anyone tell me what the viewing glass that can be seen below the cover through the inspection hole is? Not in any of my literature and search here didn't show anything most likely because I wasn't using proper terminology. Thanks.

 

[Spktyr]

I'm not sure what it is, but usually such sight glasses are part of the air conditioning system:

https://community.cartalk.com/t/a-c-sight-glass/94249/18

https://www.youtube.com/watch?v=dskuSNzpQ0s