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It looks like a mark 2 with the metal trigger, early '80s. Value of the .22 model in good condition with open sights is probably between about £120 and £150 so if it's got rust that will leave pitting when removed you need to knock some off that valuation. The synthetic piston seal may have crumbled and need replacing. They did get dry with lack of use and disintegrate. Do a search of the archive for instructions on replacing the seal. Seals are cheap to buy on the auction site but you really need a spring compressor to remove them because the mainspring is very long. Power should be a little higher than the HW35 when in full working order.