Two later medieval folding knives.

Two folding knives from the high medieval period:

A folding all-steel knife based on a 15th century original sold by Hermann-Historica auction house in April 2008. (Hermann-Historica Auction #54, Lot No.4605, "Gotisches Klappmesser") - please see image #3 for an image of the auction archive.
This example has been given additional detail decoration, in the form of the blackening of the all-steel handle, and a forge-blackened fuller line inscribed just below the spine of the blade.
The blade is forge-worked from O1 steel, the handle plate is modern mild steel.
Overall length, 230mm open, 165mm closed.

A folding knife based on an archaeological find from Dordrecht, western Netherlands. Please see image #4 for photography of the original. 
Dated during excavations to the mid-14th century, this is a style of folding knife entirely appropriate from the late 13th to the early 15th century at least, and the simple style of blade could easily justify a date of at least a century either side of those dates without much difficulty.

This example has been resized to make a smaller version, which is 100% legal for UK knife laws. Forged from O1 carbon steel, with a 62mm blade length when open.
The handle is carved from fine-grained beech, treated with linseed oil.
Overall length open: 152mm. Closed, 115mm.