How to recondition old rubber seals

Twofold theory:

Perhaps OSB is cheaper at the moment for this builder than ply and wants to use OSB as much as they can. OSB and Ply are apparently seen as the same in terms of performance and code see: http://bct.eco.umass.edu/publications/by-title/choosing-between-oriented-strandboard-and-plywood/

However, OSB, when cut, is more susceptible to water damage on the cut (the cut exposed raw wood not covered in the bonding agent/glue). Once OSB edges get wet, it expands like a sponge...which would ruin the roof.

So, it looks like they might be using as much OSB as they can, but have to switch to ply whenever cutting.

That looks like asbestos - the age would be about right. I'd get your local authority to have a look. They'll be able to advise on what you can and cannot do.

Asbestos is "safe" as long is at remains in one piece. It's the dust generated when it breaks up that's dangerous. If (and it's a very big if) you could remove it without it breaking then you'd be safe. However, that's unlikely to happen.

It will depend on your location but it can be expensive to have asbestos safely removed and the usual approach is to leave it in place if there's no danger of it breaking up.