Head rests were available as an option in the Japanese Sunny Coupe [KB10] but they were not offered in Australia.

You will find that Datsun 1200 head rest along with 120Y coupe head rests will fit straight in. Australian Sunny [B310] head rests look like the 1200 ones, but the shaft spacing is wider.
If you fit head rests, just be carefull as the holes in the trim may not be located directly over the hardware inside the seat, so have a little feel around with your finger first.

Those metal fittings at the top of the seat originally had plastic plugs in them & surprisingly, they seem to be still available. I bought four & now will not be using them.