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I think "left" is actually a present participle and means the same as "remaning".

There are just a few of the 'wireless telegraphs' that the factory produced remaining in the world.

BTW, this...

This is termed an existential sentence:
existential sentence - definition and examples of existential sentences

"There" is called a dummy subject (not the real subject.)
I think you are...

"There are just a few of the 'wireless telegraphs' the factory produced left in the world." = "There are just a few of the 'wireless telegraphs' the factory produced still in existence."