Beyond that found in the instruction manual. Serial number 136354.
It is basically an enclosed 20sec transit with built on sliding eyeglass to read a glass vernier.
Great for dusty conditions like construction.
Used similar gun with top mount edm back in the early 80s. There was only one guy among three crews that could use it good enough to close a traverse.
I did not like the sliding eyeglass. Was too easy to move off of mark by simply reading the angle. I guess that is why it is only rated at reading directly at 20 seconds.
The better Leitz micrometer type glass vernier on TS from that era can be read directly to 3 seconds.
😉
That would be the Sokkisha TM 20C, first 'theodolite' I ever used, big step up from the old jap Dietzen vernier transit I started on.
From the 1988 Lietz Catalog
30x magnification
3" Resolving Power
1.3m Minimum Focus
1:100 Stadia Ratio
Horizontal Circle 20' Graduations, 20" Vernier Reading
Vertical Circle 30' Graduations, 1' Vernier Reading
40"/2mm Telescope Level Vial
90"/2mm Plate Vials
10'/2mm Level (Circular Vial)
10C was the successor to the 10B Transit
Original version had tapered bronze/brass vertical axis. (Weight of EDM on this version often led to premature wear of the axis requiring resetting the clearance between verniers and circle.)
Later version had straight ball bearing axis like theodolites
Trough type compass built into standards when introduced.
Separate detachable trough compass later (maybe after 1980)
Clearance between verniers and glass circle had to be minimal to prevent parallax when viewing through magnifiers. As commented above it could still be an issue if you didn't have the magnifier centered where the reading was made.
This instrument similar to Dietzgen 6140/Topcon AG2 (Can't be absolutely sure of model designation).
Photo in catalog has serial number 18746 but that means it's newer because Sokkisha reset the serial number odometer sometime in the 1980's.
Information about your instrument might be available from Topcon Sokkia. Try this link:
http://www.sokkia.com/general/contact_us.aspx
Discontinued early 1990's in favor of electronic theodolites.
This one has a very large mount on the tube.
One of the magnifying eyepieces has broken loose and I may have that repaired if I choose to ever have it serviced.