Ok. Thank you for your reply. Here are my Queries.
1) When the bezel of the illumination window is rotated to a certain angle, what look like 2 sockets for electrical wander plugs are revealed. I am guessing that these relate to an alternative electrical illumination system. Is there an internal electric bulb which is powered through these contacts ? I have tested with a multimeter; one socket is connected to the body of the theodolite, the other appears to be connected to nothing. Maybe there is a switch somewhere, or perhaps the bulb has blown, in which case how can it be replaced ? It is possible that the sockets simply receive a plug-in attachment and are purely mechanical in function ?
2) On the left-hand vertical support for the trunnion there is what looks like a locked adjustment screw, which in my instrument shows signs of rather rough usage. What is the function of this adjustment.
3) On the lower part of the instrument (the tribrach) there is a threaded nipple with a central screw-driver slot. What is the function of this ? Should it have a plastic cover ?