3.8 Valve covers

I have a 1992 sable and the valve covers have been leaking for years. I finally went to fix them. After getting that damn upper manifold off, i got
the covers off and bought $40 rubber gaskets from NAPA. After looking closer at the covers, I could see that the front one was cracked in a couple of places, and both of them were warped and swollen at the sides. After looking for new covers for a few days, I ended up at this junkyard in tampa called U-Wrench-It, where you can pull your own parts. I started with some Taurus, and the front cover was fine, but the back one had that same swelling problem. I then went to a Thunderbird and the upper manifold was already gone, so i went after those. The back one was decent and the front one was swollen. By the way, these all seemed to be aluminum. I tried one more car since my ride wasnt back yet, another Thunderbird. The uppermanifold was still on but since it's RWD i could access the back cover pretty easily. I took it off and it was in perfect condition and made of steel. So i decided it was worth the trouble to get the manifold on this one off, and eventually got the front cover, also in perfect condition. So I am wondering, are these factory covers? If so when did they switch and was it because of the swelling? I noticed the aluminum ones have the Ford logo on it and the steel ones dont.
Oh, we also got someone to order us valve covers from a junkyard, but they were from a 1994 Sable and the covers had the same number of bolts, but the gasket was completely different and the bolt holes were much deeper. (the 1992 ones have a flat flange).
Any info would be appreciated. Thanks.
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