Round "button" on top of reefer roof

Dean Payne

I have an Atlas 36' reefer lettered for Cudahy in the "Sunlight"
scheme. However, centered on the roof, next to the running board is a
round "button", which I at first thought was a leftover part of a
sprue gate, or something. However, when I examined it with my
OptiVisor, I found that it was not attached to the running board.
Perhaps it fell on the roof during assembly and accidentally adhered?
No, prying with a hobby knife didn't dislodge it, it appears
purposely attached. A trip to provided the following photo:

It also has the "dot" next to the running board, right in the middle!
I have another Atlas reefer or two, and they don't have this. What
does this model? I've never seen a reefer with this feature.
Dean Payne

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