I'm quite surprised that there is no exchange token in the get orders response, since it is an unique identifier from exchange side, many of us were using that for development purpose, please do the needful.
We introduced the instrument token because there were instances that exchange token is being reused or two instruments of different exchanges were having same exchange token. You may use the same or find the exchange token from the instruments master or you can also use primary key combination of exchange name and tradingsymbol.
You may use the same or find the exchange token from the instruments master or you can also use primary key combination of exchange name and tradingsymbol.