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Which air boot is the one I want, the left or right? The right looks bigger but the left looks newer, do I want the bigger air boot that looks to be well used or the smaller one?

Instead of searching for an old obsolete air striker off a KX 250, try sourcing the newer version from a 2011 ktm 150sx for example. It's modern equipment in 38mm with no power jet circuit and no TPS. They sell on ebay for around $220US but you may be lucky enough to find one cheaper.