Hi, it is quite possible that it was for use in a quality control as a comparator. Either it may have been to match parts in pairs or set amounts, or to check on production runs, either way the operator would not need to know any measurement as such, but would set the test machine to a master and compare the tolerances for production runs, hence the zero being in the middle of the stroke.
Just my thought the reason that it doesn't indicate mm or inches and the fact the little 10-0-10 dial would suggest it being used horizontally..
Regards Nick.