Hard to tell for sure if it is original as there were several plants that produced these including 1 in the U.S. so any of them could have done it at the factory by request of a customer, but most likely it was added by a previous owner through a reno...the window itself could tell you with any identifying markers by contacting the window manufacturer...i.e. if it is a early 1970's trailer but the kitchen window is a Hehr model manufactured in the late 1970's...also you could look at the rivets and see if they match the other windows
, but they could have all been changed out at some point also. If you carefully examine the fine details of the windows
and the area around as to how it was installed you may be able to make a pretty good assumption as an answer to you question.
I contacted Hehr and gave them number of both of my jalousie and both were made in the 1971 era of my Boler
, but the driver side window was larger and had tell tell signs of being installed after the factory production as the original kitchen upper cabinet had been trimmed to accommodate the larger window.