Can anyone tell me how to remove what ever it is that stops these reels from back winding or a link to a diagram.

                                                 thanks

removing the fixed anti-reverse trap (pawl):
 
requires the strip down of the reel in order to remove a tiny brass triangular plate thats sits on a peg under all the cogs.its been a while since i've done one so i carn't give a detailed description of the top of my head but all i can say is lay out each piece as you remove it on a piece of paper untill you get to the tiny part then replace all the bits in reverse order.
From memory
1.Remove front cover, cup and spool
2.remove handle and bearings
3.remove side cover plate remove 2 small ciclips that retain the oscillating yoke with draw the device of the cradle pins.*
4.unclip the tab washer at the front of spindle and unscrew the spindle.
5.pull out the lift cradle
6.remove the clutch plates and gear inside main body the anti reverse pawl will come off its pin with the gear slide it off the gear and re assemble.
* just watch the two little circlips dont fly away when prising them off.A small screw driver with magnetic end is pretty good for levering them off and cover the housing with a cloth or piece of kitchen toweling whilst prising the clips off.You might find it usefull to draw a few pictures whilst stripping it down or place the parts in order of removal.

any problems just ask (I have it all in pictures if you need them I was going to make a tutorial movie to go on youtube ,but ain't finnished it /edited the sldeshow yet .might make a pdf instead ,I'm still looking for a abu 503-8 manual /instruction booklett to go with it !.)