A couple of other manuals can be found at
https://www.sil.si.edu/DigitalCollec...0203/index.htm and
https://www.manualslib.com/manual/10...15.html#manual
At
https://www.manualslib.com/manual/10...?page=8#manual you can see how to regulate the pressure. There were a few somewhat different pressure controls, but similar. Either a lever or sometimes an outside knob that could be pressed to release the pressure and then press down on the center to increase the pressure.
Just an interesting note about the Smithsonian manual is that while it does say Singer, it isn't threaded through the needle like the Singer 15's at that time frame. The
Singer 15-90 manual will work for most things, but it is threaded through the needle differently. I like the 15-90 manual for showing the oiling points.
Janey - Neat people never make the exciting discoveries I do.
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