This indicator set up was made to compare inverted dovetail
slides for parallelism when scraping.  Probe end and indicator
end holders swivel, and have a limited range of up/down slide
for positioning in the slotted bolt holes.  Thus the holder can 
also be used to compare measurments across various angle dovetails, 
or square sides such as box ways.  Probe end uses swaged in
5/16" ball bearing.