This crescent shaped piece inserts into a trumpet's third valve tuning slide ring, significantly reducing the empty space distance the left ring finger needs to travel before it begins kicking out the slide. This may be particularly useful for kid-sized hands, which often have trouble fully extending the third valve slide to get those Ds and C#s in tune.
Sized for a 11.3mm inner diameter ring, which fits at least my Bachs (37 and 65) well.
The author hasn't provided the model origin yet.