Hi cmpman
They look like 105mm Light Gun L118/119 breech blocks, compare them to the 25 pounder block posted on the thread. That has a precussion lock to fire the primer. The light gun is electricaly fired by a fireing box and Peizo ignition so the "fireing pin" is in fact an electrical contact which withdraws when the breech is opened (or it will be broken off). Contact is passed through a series of contacts in the block and breech ring from the box to the fireing contact, if these don't line up, as the breech is not completly closed, the gun wouln't fire, we call this a built in safety.
I hope this helps, the last time I worked on Light Gun was 9 years ago!!
Regards
Artifficer