Saddle Bag / Scrambler Bag. Great quality. Very sturdy, but made of thick and very supple leather.
The bath is not too big, but you can certainly use some tools, some rain gear or other things.

Dimensions:

Max length top: 300 mm
Height max: 200 mm (diagonal slanted wall)
Height min: 150 mm (diagonal slanted wall)
Thickness: 70 mm

Assembly goes through 2 straps on the back.