Metal behind plaster on chimneybreast

[AGriffiths]

When trying to hang a large picture above a 1920's fireplace I encountered metal behind the plaster. There is no liklyhood of wires, gas pipes or plumbing in this location.

Any idea what this might be? Whether it is localised and drilling possible above, below or on either side? Or if it is safe to take a metal bit and go through it?

[raysparx1]

Could be old gas pipes or metal conduit (tin whistle) that was used for electrics years ago but I would think it probably the old pipes feeding gas wall lamps, I know you say it isn't gas or electric but most houses that were around at that time had gas wall lights, other than that I really can't thnk what else it could be.

[Fitzer]

Could be an access hatch to the chimney for servicing/cleaning of the flue, someone may have just plastered over it.

[SpamBammer]

could it be a bomb!