Received the firing assembly from Cz.And it does fit in a ZKK600but not without some work.Take down to remove the spring is the same as a ZKK600,but that is where the similarity ends.On a ZKK,after removing the spring and retainer nuts with the tool the shroud slides right off the firing pin assembly .Not so with a CZ550.After removing the spring in the same fashion as the ZKK,the shroud of the CZ550 is held on with a donut plug pressed into the rear of the bolt shroud.The donut plug must be removed before the firing pin can be removed.What rocket scientist thought of this.Difficult job removing but finially got it done.The firing pin is the same in the front but the cocking pieces are slighty different.On the ZKK, the cocking piece is pinned to the firing pin.The cocking piece is silver soldered or epoxed on the CZ550.Also on the ZKK a small retaining bushing is fitted over the rear of the firing pin that sticks out about .3" to the rear of the cocking piece and is peened ,polished and blued/parkerized for a finished look.On the CZ this is hidden under the large cumbersome bolt shroud so when fitted into a ZKK shroud the .3" of firing pin sticks out behind the cocking piece.So you must make your own bushing,but this has no effect at all on the firing of the weapon.Also the top of the CZ cocking piece is slightly less in diameter than the ZKK so there is a tad more space between the cocking piece clearance in the inside of the bolt shroud.Again unless you miked it as I did you will not notice it.The working parts of the cocking piece are the same.Trigger pull was excellent and the firing pin potrusion was about .055; the same as my other ZKK rifles.The bolt takedown device also works perfectly with the CZ550 firing pin/cocking piece in the ZKK600 bolt shroud.