I've not seen one on an LM27, but such an arrangement is called a 'horse'. They come in various shapes and constructions, and are common on working and leisure boats, especially older, smaller and non-racing craft.
Because the LM27 mainsheet track is very short, I would not have thought it would suffer any great performance penalty compared to a track. I'm not clear from the photo whether the horse is angled forward to best take the load from the mainsheet.
