I suppose it could stem back to the days of part radar, part procedural, when you could maybe have locked an A/C onto a particular heading when it was tracking to/from a VOR without recourse to radar,but I honestly don't know how that would have worked without some radar back up. Maybe one of the older members can enlighten us - Alzheimers permitting.
I'd think 'anotherthing' is right.
I started ATC OJT from behind the Procedural Flight Progress Board. When I got my licence the system had meanwhile switched to full radar service.
In procedural times, a radar man (late sixties men only, women's lib still in its infancy) was available in a darkened part of the opsroom, usually feet on the table and sipping his umpteen's coffee. He had to be asked via intercom to solve or expedite a problem. When he came into action he used the same frequency and probably distinguished his heading instructions from the procedural clearances by adding the word 'radar'.
Procedural controllers may only clear a/c via flight planned route or significant points. Vectoring instructions may only be given by radar control. Positive control, as the Americans said, I have always liked that term and regretted we didn't use it in Europe. Main vectoring instructions prescribed by ICAO PANS/RAC are:
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'continue heading (three digits)', which implies that the controller has to ask the heading first, otherwise he wouldn't know which digits to continue;
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'continue present heading';
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'fly heading (three digits)'.
Uncertainty might arise when the a/c is transferred to annother freq and has to report his heading to the new station. Since each aircraft is always on a heading, the receiving controller will not know whether the reported heading was instructed by ATC or not. The term 'radar heading' makes that clear. It may not be in the books, but imho it precludes a misunderstanding or worse.
Admittedly it gives rise to forum discussions. But that's better than a discussion on the frequency.