This block is the ratchet block for the mainsheet in all new Flying Scots.  With the new racing Flying Scots it is also used as the ratchet on the seat for the jib sheets.

The Carbo Ratchamatic® is a load-sensing ratchet block that rolls freely in both directions under low loads and automatically engages the ratchet as loads increase. Shifting between ratchet and light-air modes is seamless.