First of all it should be said that I'm sorry for focusing almost solely on the history of Levi's. There are of course other iconic denim brands that should be focused on, but it is the history I'm most familiar with and it is the one denim brand that seems to have the widest appeal. I may be focusing on the history of the two other big, old American companies - Lee and Wrangler.
Levi's has a big gap in their knowledge of their own history from the very beginning of the company to around 1906. In 1906 San Fransisco was hit by an earthquake and big fire followed. Everything was basically lost in that fire. This has been one of the things that their very own archivist, Lynn Downey, is trying to piece together. She has been collecting old clothing and memorabilia (of literally all sorts) from the company and trying to piece their history together. This collection is unfortunately not open to the public, but you may find reproductions of some of the pieces, as the line "Levi's Vintage Clothing" is mostly based on things found in the archive.
The oldest pair in line of what became the 501 jeans in Levi's archive - or should I rather say Lynn's archive - is a pair of jeans from around 1879. This pair is locked up in a (fire-proof) vault in San Francisco and they have a value of approximately $150,000. Not bad for a pair of jeans that were bought for $1.50, when they were originally purchased.
If you want to expand your denim vocabulary some of the features on this pair are, natural dyed indigo denim, single stitched construction, hand punched copper rivets, cinch back, one back pocket (with hand done arcuate), raised watch pocket, no belt loops, but suspender buttons and a button fly.