How badly do you want the pan? That's what it is worth.
The only Griswold pattern I have ever seen is the No.0 skillet pattern pictured in the L-W Book Sales Griswold (first volume) book on p.69 and on rear cover.
The skillet part of the pattern is made of aluminum. I don't know what other Griswold pattern were made of. Molding sand is packed around the pattern to make the MOLD in which the iron is poured. The mold is used only once as it is broken to remove the casting.
A pattern has to be fractionally larger than the finished piece to allow for the shrinkage of 1/8" per foot when the casting cools. That brass skillet should be about 3/16" larger than a standard No.13 skillet if it is a pattern or part of one. If it is the same size I would think it is a prototype to be send to the pattern maker so he can make the necessary pattern to duplicate the prototype. I bet that is what the piece is; a prototype since a pattern is not a skillet but just parts to pack the sand around to make the mold which will then make a skillet.
So, to recap, the pattern would be aluminum, wood, iron, brass, plastic, etc. (though I don't know what Griswold used-probably aluminum, brass or iron).
The mold is made of molding sand.
The skillet is made in the mold which is made from the pattern.
What we need for a field trip at the first WAGS convention is a trip to a foundry to see the way they make castings. It is fascinating to watch but my experience is limited to a local steel foundry that makes truck suspension castings and I have had to watch the process from outside the door (thanks OSHA and insurance companies). When that electric furnace with it's 6" diameter carbon electrodes gets going you have to cover your ears from the noise. Beautiful!!
Steve