strange old ceiling box---can I hang a fan from this?

Trying to see if I could mount a fan (something I've done before), I came upon something I've never seen before. It's an old round box on the plaster ceiling, maybe 6" or 5-1/2" in diameter. It's shallow (almost a plate rather than a box) but made of heavy-gauge metal. There is one maybe 1/4" female thread in the center, and that is the only way to attach anything to it; the existing light fixture was attached to a threaded 1/4" post / nipple that was screwed into the center of the box.
There are three round holes on the top (horizontal) face, and rigid conduit comes vertically into those holes. There is clearly a stud next to this box, but I cannot tell if the box is attached to it. It seems very solid and stable compared to modern drywall boxes, though.
So can I hang a fan on it if I can figure out how to mount the fan using the center thread? I can't remove it without major damage to the surrounding plaster an moifications to the rigid conduit.
Many thanks for any suggestions!
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APJ wrote:

out a hole into the bottom/top of a new metal box and then you attache the new box to the old box via a theaded rod with a not on the end to hold the new box and attach the fan to the new box... Or get up in the attic and look to see what is holding the old box onto the stud????? and remove it if you can and put in new metal box....
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APJ,
You must determine how this box is supported. Do not attach a heavy load to it without doing this.
Dave M.
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those old style plates [everyone I know calls them DO HICKYS} are for BX cable, or I guess conduit in your case. and are only heald up by one screw in the center about 2" long, that is screw into a beam . you shouldnt hang a fan from them.but they are a pain to get out without damage becouse they are often plastered in. and the screw slot is full of plaster.but it canbe done with a little effort and the new box canbe mounted rite to the beam that is there,get a shallow box and chissel out alittle from the beam to get it flush.

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