Keyhole fittings under altar mensa

I am making an antepedium for my church. We have discovered a series of keyhole brass fittings and wonder if they are to hang the antepedium.
They are flush with the underside of the thick granite slab that makes up the altar.
They clearly have another component something like a door chain. A button of some sort goes into the round hole and is slid into the rectangular channel.
I’m looking for some information on the missing component and how it might be used to hang a fabric frontal.
Has anyone seen this before, and perhaps have a name or picture of it in use?
The Church was built in 1960.
Thank you in advance for any information.

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