This cover is the latest S3 Head for GAS GAS TXT 250/280/300 with the horizontal water hose outlet.
The various inserts for this head are also available to buy separately.
If you want fairly flat power for beginners or for really slippery conditions then you would set the bike up with a “low” compression insert. If you want to keep the bike standard, order the “standard” insert. If you wanted a more aggressive power setup, or you are going high in altitude then you could experiment with a “high” or “very high” compression insert.