Could it have been the base for an approach aid such as PAR. They were swung onto approach heading once and may still be to this day. The circle would have been for running wheels for a unit mounted on a centrally mounted plinth.
That certainly gets my vote. I can remember one such (ACR7?) which was IIRC a
Runway Alignment Aid, ie, better than an SRA but still without glidepath information.
On one grovelly weather occasion, I was asked to hold while they turned the kit round to get a downwind approach with better cloudbase!
But since when did a downwind approach ever worry a rotary mate .......
