How to tell you have OEM inner fenders - maybe
 

When I pulled the RH inner fender I found "they" had run a sheet metal screw into the outer fender instead of using the fixed nut in the "pocket" of said fender. Made me wonder, and it was suggested that is it was a replacement. Since then I did somewhat a dry run with the fender and inner fender off the truck (using a large exercise ball), and I see no reason why they did this outside carelessness.

The fenders were stripped, and I left them in the garage (don't do that). They were supposed to be done by now, but that's another story. Anyway, water got on them bringing rust, and I cleaned the worst of the two first. I used a rust remover concoction on the entire thing and sprayed it off with water DEFIANTLY DON"T DO THAT. I didn't want to leave it on there, for fear it would etch the metal. The result was a solid rust color (go figure). So I spent much time getting familiar with that fender.

I was a bit excited to see my RH inner fender had two stamps (pictured). Anyone ever seen these stamps? But to my disappointment, the LH fender doesn't have a single stamp. Couldn't tell you, but 100,000/1 the LH is original. I could be wrong, but I still believe they're both OEM. The reason is the braces spot welded onto the fenders are exactly the same. Even the holes are equally off-center where the brace is located front bottom inside.

On a side, this cleaning rust business wasn't a total waste. I discovered lacquer thinner and a scotch pad is your friend in these cases. The thinner breaks up the surface rust and comes off easy. I spent well over two hours cleaning up the rust on that RH fender. Took 15 minutes to do the LH fender.

I now keep them in the back room.

Re: How to tell you have OEM inner fenders - maybe
 

Braces spot welded.

Re: How to tell you have OEM inner fenders - maybe
 

Just checked out my other Chevy. It's assembled and painted/rusted, so I may be missing something. I see a stamp on the RH inner fender as well. It reads "L8" at bottom outer rear. Same location as the RH inner fender in the OP, but reads differently. No sign of any stamps on the LH inner.