The rear seal takes all the pressure, which I think I mentioned earlier, while the front seal has an easier life. If you are careful then you may be able to exchange front and rear provided the seals can be worked free by lubricating where they sit. To do that you can submerge the entire piston in warm oil for a day or so as then oil can go places where it would never go before. On a multi-piece piston they can be disassembled around the seals, however if the seals have "reverted", i.e. the rubber softens and loses its mechanical strength, then the seal is kaput! The Metzeler company (http://www.metzeler.com/site/com/) are still in business and were associated with Nemrod as both companies names are listed on the patents for the later pneumatic guns such as the big tank Nemrod "Mariner", so may be worth asking them for a lead.