Where your ring was, I have a small set screw to limit the rise of the lower stage.
I refer to to Fine Focus (lower) and Course Focus (upper) but the actually travel at the same degree of movement along the chrome bars, it is just that I can not use a grain focus and reach the upper knob at the same time, so the lower control has, by default become the "fine focus". I would guess with that extension arm, it is more convenient to use the upper as the fine focus when looking through the grain focusing loop?
My 138s came from a lab too, and the negative holder has a slot for 35mm by about 4 inches long. I am guessing that it was a custom made item to suit their needs, I have not even tried to use it to date, but one day I will try to print a four frame 35mm strip enlarged to 14 inches across, just to see how it looks.