Kirk;
The bolt is the part which holds the barrels closed. This would be the part which goes in that round hole & extends through to engage with a notch in the lug beneath the barrles. Not familar enough with the Meriden to say for sure, but I believe it has another bolt in the top which latches into the rib extension.
In dis-assembling the locks, you can make a simple little cramp (or two of them) by sawing/filing a notch in a small piece of metal strip. The hammers should then be pushed back to full cock, the cramp placed over the leaves of the spring to secure it. The sear is then released. This removes all spring tension from the hammer & the lock can then be dis-assembled without parts flying around. The spring can be left cramped until re-assembly.
