Hi Graeme and Gene,
The lower one is very similar to one of mine, which according to Elgood is from the Asir/Hijaz region, and dating to the 20th century, Bani Malik and Bani Shahr Tribes.
The upper one appears to be from the Hijaz region, and my guess would be 20th century.
As far as the blades of these particular long bladed Jambiyas are concerned, I have always believed that the flat blades are of earlier vintage than the ribbed ones, but that is only my opinion based on the viewed age of the total piece. I have used as "rule of thumb" that flat blade is 19th or very early 20th century and ribbed a little later. I have nothing concrete to base this on other than gut instinct. Perhaps others here can elaborate.
Nice pieces Graeme.
Regards Stuart