Sorry I cant really help you with this.
The locking piece is similar to the vz.58 (only much bulkier). MCM converted the open bolt design to a closed-bolt and added a spring driven striker piece that hits the firing pin when the striker piece is driven forward into the back of the modified bolt carrier. To the best of my knowledge none of these parts of the action can be adjusted to change head space. Having said that, if a different bolt is swapped into the action -- (say a new bolt replacing a bolt with a worn bolt face) -- that may change the head space. So knowing that the action is somewhat similar to the vz.58 in regards to the locking piece, one could assume that a set of 7.62x54R go/no-go gauges would work to tell you if the bolt is closing properly on the barrel and if the headspace is out of tolerance. Put in the gauge see if the bolt closes and and check the position of the locking piece.
Side note: NEVER NEVER NEVER charge the MCM UK-59 without a barrel installed. The locking piece can damage the receiver.
I do know that depending on the barrel the head-spacing procedure on the UK59 can be an undertaking that requires a lot of modification to the top cover which cams into the barrel. My top cover was modified extensively to set headspace.
The gun was sent in to Marcolmar to headspace, twice. Once because the headspace was off with the original light barrel that came with the gun, and once because I found a new heavy barrel. The first time it was headspaced the I believe modification was made to the light barrel and not the top cover. The second time considerable modification to the top cover was evident.
Probably because of those modifications original headspace modifications, the gun required a new light barrel when both the heavy and light barrel were sent to MCM to headspace. Both were matched so they could be hot swapped.
Cost from MCM was about $200 ( if I remember correctly ), for the headspace work that worked with the two barrels. They didn't charge me for the new light barrel. The guns top cover had been greatly modified -- as in material was taken off the front. and you could see heat was applied as the paint had changed color (perhaps from grinding). (I saw no welding. Mystery work.
If you would like I could send you pictures for clues.
Sorry I couldn't be of more help. Please post any update if you would -- always interested in this sort of information. Best of luck.
Pete
