I see Mod 280 in the description. The one with four leveling screws could be a modified version of this:
From a 1966 Brunson catalog. Brunson often modified their standard offerings to suit a users application.
Without the mirrors these can't be worth much.
So, would you turn the mirrors to the angle you wanted, and superimpose the backsite, and the forsite together, and then they were at that angle?
N
I think this might be used in optical tooling applications where an angle other than 90 degrees needs to be set. Most optical tooling measurements are establishing parallel lines and perpendiculars to those lines. For example (this is the only time I was on site during an alignment with my employer many years ago) paper mills have many rollers turning at high speeds. If the rollers are not parallel to each other paper will go flying every direction. Establish a reference line with one jig transit and then set up a second with first surface mirrors on the horizontal axis so you can autocollimate the telescope of the second instrument to be at a right angle to the first. Then sight the near end of the roller and the far, set roller so it's centered on the cross hairs of the instrument at both ends and it's at a right angle to the reference line. Also check height.
Looks like you'd establish your reference line with the 280 by autocollimating and then turn it to the angle desired using the circle and verniers then use the jig transit to autocollimate and establish a reference line.
Brunson has good information about industrial alignment on their web site if you are interested:
http://www.brunson.us/p/LearnCenter.asp
High accuracy total stations, laser trackers and special application software have changed industrial measurement methods in the past 10 or 15 years. Users and vendors have an organization that meets every year:
This is one of the software companies, New River Kinematics: