Actually Sierra Pacific has been around a lot longer than 1976. Look at the dates on some the photos above. They used to operate scheduled flights out of Mammouth Lakes CA (Ski Area) with the Convairs back in the 70's.
Over the years Sierra Pacific has operated some Casino charters for to Lauglin NV. Also back in the early 90's they flew scheduled service for Morris Air before they got their own operating certificate. I flew them on a Morris Air flight from SJC-SEA back in 91 or 92.
They do lots of Forest Service and BLM charter work during the fire seasons carting firefighters and their gear to the hot spots. I see the 737-200 regularly in Alaska during the summer and they can also be seen in Boise, undoubtedly doing work for the Interagency Fire Center.
I also JUST saw one of their planes parked alongside a Southwest 737-700 in SLC yesterday.