Well, I haven't seen them since around '72. I raced with Lee Smith for a year or so, CRAZEE CUDA. He had a aluminum set of dual spark plug heads on his Hemi, with a very thin stamped set of aluminum valve covers on it. The holes for the spark plugs were elongated, just like your part, that part sat down into into a slightly depressed area, and the spark plug tubes went through them. That why they have a thin rubber coating on them. As I remember, they or the valve cover didn't seal the best, but got the job done.
I may be wrong, but think that Lee was responsible for starting the hemi dual plug thing. He had his own home made dual plug dist. Lee's prostock car was sponsored by Chrysler. The head had another hole machined in beside the orginal spark plug hole and another spark plug insert was tig welded in. I do not have a picture of the set up. The ones that I do have, are of the single plug set up, with DuCoil distributor. The hood on the Cuda had a round bump or relief in it for the second head on the distributor.
Here's a picture of the car, second picture is at the Cordova dragstrip in Illinois, that's me standing. If you look closely, just below the left edge of the scoop, you can see the aluminum bulge for the distributor head.
Someone else here may confirm this part. Maybe up with the racers. Would be nice if you could find a pair of the valve covers that they were used in. Someone out there has them.