the Canvas ones were impregnated and water proof, but then I am talking 30 years ago. age has removed the proofing for sure! Yes, a try first is a good idea.
I had written this on several LK 35 threads before, so I am like a parrot, but the design is horrible for an average height person. The frames are made from thin steel tubes that dig in the back painfully, they are too short top to bottom so it basically does not fit.
a hip belt can be easily attached, but then the frame sits low so the shoulder straps hand behind the shoulderblades.
We used them fully loaded.
The vast of us, both career and National service, bought proper back packs instead.
I guess they are good enough if you carry only a few kilos a few miles though!