Later Sky Chiefs had those inside, so that seems right. Most had the Venus 178 until Breitling began using them up in their Navitimers in the mid 50's, so Benrus went to Valjoux. Here's a good overview of the Benrus Sky Chief movements/models: http://mansfinelife.com/2014/03/26/watch-collectors-notebook-benrus-sky-chief/ ...and a quick pic of my ‘40s Sky Chief and its Venus 178:
My 1501 with perfectly patinaed blue sigma dial! (I will change the date... after I stare at that sweet sweet open 9 for a few more minutes!)