Place stamp in a decompression chamber for several hours?

But seriously, once the paper fibers have been stretched in one direction I am not sure they can be 'rebounded' or reclaim the actual original shape.
Obviously you could wet the stamp and straighten it back out but this would only be stretching the fibers in the opposite direction. In other words, doing more physical stress on the fibers. I would leave it as is and assume it was originally wrapped around a bottle?
Don