If it is a single phase transformer you could temporarily disconnect the secondary, and by ohm meter test for the most resistance between the primary winding and start by feeding in the 208 in there and see what the secondary comes out to, reduce to lower tap until the correct secondary voltage is seen.
I know it is not the best scenario, but in the absence of any other info, it may come down to this?
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.