We are in rural Australia and have moved into a house with this fireplace.the house was built mid 1960s and we are at a loss as to what the ducting and vents are for or how we use them
This type of fireplace is a pre-fabricated fireplace. Commonly used to simplify a fireplace construction. The vents on the sides allow air to enter a cavity thats surrounding the box that actually holds the wood and fire. The air is heated and comes out the holes on top. Theres nothing yor you to do with them. I've seen alot of these and I've rarely seen them work like its supposed to. Or as good as you'd think. Many, many people who have this type of fireplace either re-brick the openings(which I dont suggest unless you can find bricks that match good enough for you). Or just put the grates back and leave them there. If you dont like tne way the grates themselves look, you can always paint them while they're off and get them a little cleaner. Flat black spray paint has been popular.
Now as far as drafts go. I get alot of questions about cool air coming in the room from the grates. Especially in the winter when its cold out. If itsa problem with you, tame the grates off and put some insulation in them then replace the grates. Dont use any foam board insulation and definately not that expanding foam spray insulation.
I hope I was helpful and if theres any more questions, let me know.