I might think in the line of a substance that came in contact with the block ( oily food or liquid particle). The bottom center stain I would say is definitely a contact stain. I have seen old rubber bands leave a similar type of stain.
More like "age spots" originally in the paper by appearing at a later time caused by oil, dirt , etc., as explained by several other posts. Do not attempt to remove, Akommar... that will damage the gum and may cause problems on the printed side.