I contacted Nason today and spoke with their technical support for the SM switch. They did indicate that it comes precalibrated/preset from the factory. They did indicate that if it is out of calibration you could contact them and they would be happy to repace it or recalibrate it by giving you an RMA. The technical support rep said that if the end user wants to "tweak" the pressure, that they will not warranty it. They said to field calibrate it, but only in a small band ~5-10psi, this is what they do at the factory if the proper base was assembled. The factory uses different bases that have different height diaphram pins for different PSI ranges. I asked that if the switch setscrew is loosened and silicone is removed is this a safety issue, and they indicated that the entire plastic top (limit switch) is removable without a safety problem. We did take one appart at our shop and it is a limit swith screwed onto a blind threded hub with a small diaphram pin that extends out of the ~. 0625 hole to push on the limit swith metal lever. They did indicate that if you desire an on the fly fully adjustabe switch that the CJ switch is the way to go
http://www.nasonptc.com/pdfs/Switches_Catalog.pdf page 13.
I could post images of a disassembled switch possibly tuesday when we have our next meeting.
Edit- I see I posted this before seeing that Mark
contacted them too, the only discrepancy is the psi vs 5 to 10 PSI.