groove 16c as will be discussed below fig3 a shows in an exploded view the relative position of the flat head 14a the flat chain 15 the coupling pin 16 and the circlip 17 while fig3 b shows these components in an actual assembled state the elastic circlip 17 is received in the slot 14d and within the groove 14c projects into the groove 16c of the pin shank 16a and closely surrounds the groove bottom after it has been snapped in fig4 illustrates a spring biased locking arrangement including a leaf spring 18 mounted at one of its ends on the flat head 14a by a screw 19 at its other end the leaf spring 18 carries a pin 20 which projects through a bore 14b in the flat head 14a and extends with its free end into the groove 16c of the coupling pin 16 fig5 illustrates an embodiment in which a bent spring 21 is secured at one end 21a to the flat head 14a adjacent its other end 21d the spring 21 has an arcuate portion 21c which lies in the pin groove 16c thus immobilizing locking the pin 16 in the embodiment according to fig6 a and 6b the pin locking arrangement comprises a yoke 22 which has a leg portion 22a provided with a bore hole 22b an oppositely located leg portion 22c having a throughgoing bore hole 22d provided with a lateral opening 22e the yoke 22 is inserted into slots 14d and 14d 39.