Rain cover for backpack, Mountain Equipment, probably for 70 liters

This appears to be a product from around 1997.
It belonged to my late father's mountaineering gear. It's a backpack cover, and although it's hard to see clearly, I believe it's for a 70-liter backpack.
This is a used item and is quite old, so we are looking for someone who understands this.
A storage bag is included, but it will be sent out of the bag to fit within Nekoposu shipping dimensions.
#IshiiSports
#BackpackCover