Hi peebee;
230/460 was a common transformer voltage years ago. it is interchangable with a 240/480 volt transformer.
It may be from the UK or EU.
Pulling a center-tap out of a transformer that does not have one will void the approvals unless done by a shop with the authority to issue approvals on rebuilt equipment. For a small transformer that would probably cost more than a new transformer, particularly if it is potted.
Also, older electricians and engineers sometimes habitually refer to 120/240/480 systems as 115/230/460. That may have been the standard voltage when they recieved their training, or it may have been the voltage current when their mentors were trained. Unless there is a particular reason to know the exact voltage, I regard references to 220V, 230V, and 240V as equal.
If the exact voltage is important, measure it. It is seldom exactly 240 Volts, and if it is 240 volts at lunch time it will probably not be 240 volts at supper time.
respectfully