The HW 55's barrel lockup is a rather different and more complex affair. It actually has TWO barrel latches - a small sprung auto bolt in the standing breech face (solely to hold the barrel closed before and after cocking), and the famous rotating bolt (to lock it firmly in place).
In this photo:
A - The rotating lock bolt which is operated by the thumb lever, seen here partially closed. Note serial number stamped on top. When fully open, the flat face you see here is flush with the standing breech face, creating space for the breech block to swing past when cocking. When closed, the bolt's curved bottom surface locks things down
B - Tip of the small automatic sprung latch bolt in the standing breech face
C - The adjustable, pivoting locking bar that the lock bolt engages
D - The auto latch engages this dimple on the rear of the locking bar
E - The lock bolt engages this small curved surface on top of the locking bar
Note the leather breech seal, which the HW 55 kept until late in its production run. The 55's breech block is slightly longer at the rear than the HW 35 or 50, so its fit is very close - the seal protrudes only a few hundredths from its face. You can replace it with the standard HW plastic seal, but it requires some trimming.
In this shot:
F - This big "wagon wheel" screw adjusts the angle of the locking bar. Turning it "out" (counter-clockwise) moves the front of the bar downward, thus pivoting its rear end upward, which tightens the lock
G - The pivot pin for the locking bar