It's hard to track down information for certain, and there are a lot of stories, some true, some not, about various patterns. I welcome any corrections as I myself am just as curious, and again... on the Internet, you can read things, but that doesn't make them accurate.
1. It would seem that spring-tensioned folding knives are themselves a fairly "new" thing, only being made in any quantity with the rise of the Sheffield cutlers in the 18th century.
2. That said, it's said that the Barlow pattern has been made in some form, which we probably wouldn't recognize as a Barlow today, since the late 1600s. Colonial-era "barlows" seem to have had a pistol grip, for example.
3. The whittler-style knife, with a seahorse frame and a pointy straight edge blade, was called the "Wharncliffe knife" beginning in the 1820s, after the Earl of Wharncliffe, who probably helped Rodgers get a Royal Warrant, permission to emblazon their products with "Cutlers to His Majesty."
4. The Congress pattern also dates to the early 19th Century. It was also devised by Joseph Rodgers & Co., and there are definitely examples which predate the American Civil War. It was probably named not for any political institution, but for the way the blades "congress," or "come together" (if you are up on your Latin.) It's said to have been particularly popular among the well-to-do in the American South.
5. Single and double bladed jackknives, like GEC's 15 and 77/78, are probably the oldest patterns, because they are the simplest and most straightforward workingman's knives. There are knives in a catalogue of 1816 that are substantially like the jackknives of today, and there are plenty of examples from the Steamboat Arabia wreck from 1857. Spear blades would often seem to have been fatter, like the ones on the GEC #85 and Eureka Jack patterns.
6. Much is said about the gradual evolution of pocketknives (per se) from the folding penknives employed by office clerks to cut goose quills for writing in the 18th and 19th centuries. Personally, I wonder how accurate this is, because it seems to me that men since time immemorial have needed a sharp blade to accomplish the basic tasks of life, and I find it very hard to believe that they only "discovered how handy a pocket knife was" after they learned to write en-masse and bought them to sharpen their quills. Either way, the increase in literacy, office work and bookkeeping, general use of money, and the rise of industrial knife production in factories, in the 19th century, all had their part to play in popularizing finely-made slipjoint Spring knives (as opposed to the friction folders that have been around forever.)
