Hi Guys,
I didn't know what this was at first, but now it appears to be a cutout for a power socket.
Does anyone have one of these spare?
Or at least a picture of one, or what type it's called,
so I know what to look for?

Image Link

Cheers, Brek.

Is this for output mains power, to an accessory such as a gramophone?

Are you referring to a single socket plug base such as this? :

http://seeq.com.au/PIC/RPG/single/11806060_RPG.jpg.

Back in the day they were shorter, such as:

http://www.restorationonline.com.au/images/5564%20%5BDesktop%20Resolution%5D.jpg.

These days we would use an IEC socket such as:

http://www.futurlec.com/Pictures/IECFEMALESOCKET.jpg.

Hi GTC, all other holes in the back are taken by manufacturer,
this is the only free cutout, and it just had a mains lead fed underneath when I got it.

The unit is a console radio, no gramophone.

I thought there might be something inside, since I have found
a similar thing, like the surface mount one you linked,
but mounted the same way inside the chassis.

Otherwise I have no idea what this cutout is for.

It does seem strange to have holes so close together
that you could short the radio end of the mains lead by plugging it in a little off centre.
The other radio with the thing inside had one large hole
in the chassis for it.

I don't know what those holes were for.

The usual method of routing power into a radio of that era was via a grommet. That is no longer satisfactory. Today we use a suitable size cable gland:

http://www.intexconnect.com/product-sheets/cord-grips-cable-glands/#.

Rubber grommets are still okay provided a cable clamp is installed inside the chassis to prevent stress being placed on connections. This is my preferred method as it matches the original installation in a visual sense.

Tying knots in flexible cords is naughty though.

With respect to the holes pictured, they could be for battery-powered models, some of which require more than one flexible cord. Manufacturers most often did not tailor-make a chassis to a specific model and would stamp out one chassis pattern to suit all the models that chassis would possibly be used in. It was a cost thing. AWA was well known for this with many of their five-valve receivers having extra blanked-off holes for extra valves, transformers, surface-mount condensers, etc.

There is no evidence that it was ever used in or outside the
chassis.. you'd think you'd still see marks from screws, etc.
but if it's not for power, then the chassis does not have any
cutout for a power cord at all, and strain relief must have
been on the cabinet, making it more difficult to get the chassis out.

I like the idea of IEC, have even considered buying a punch.
It might be butchery, but for the right IEC cutout, you get an
Earthed connection, fuse holder, and power switch all built into the socket.