Abu Garcia Thumb Rest (Casting Control Bar) #23840.  This Thumb Rest is used on the 5600 Seriesa AB(Anti-Bcklash) casting reels. There is a small plastic slide bar incorporated into this thumb rest that “locks” it into place in the frame of the reel when not using the “anti-backlash” function.  This bar is the “weak” spot in this design as it easly breaks making the normal use of these reels not avalible.