Will,
I don't know if this bears any resemblance to the internals of your tester but perhaps there is a later Patent registered for yours with the adjustment details.
http://worldwide.espacenet.com/publi...m&locale=en_EP
If you move on to the later pages in this Patent, it gives a detailed description of the mechanism and adjustment.
There's also a document outlining the use and calibration of Rockwell testers from Wilson. I couldn't find a link directly on their site but it can be downloaded as a pdf via
http://www.instron.us/wa/library/Str....aspx?doc=1154. (Copied from the references (#9) on the Rockwell Testing page of Wiki)
I'm in strange territory here but I hope the Patent search idea might assist in some way.