What lies inside. I'm not sure what the purpose of the dished plate on the distributor side of the diaphragm is for, the 'as rest' position of the diaphragm is controlled by a step at the top of the actuating lever resting on the inside of the lower half of the case.

The 25D vacuum module. The a ridge on the spring-clip engages with the notches in the knurled wheel to give 'click-stops' for each fraction of a degree of adjustment. The coil spring steadies the vacuum module in the distributor body and prevents vibrations from causing small variations in timing. This is a later 25D with push-on port for rubber and plastic pipe, earlier versions had a threaded boss for a copper pipe with a separation chamber at the carb end. There should also be a small circlip to fit on the end of the threaded rod to stop the knurled wheel turning beyond the fully advanced position.

The 25D module adjusted to the fully retarded position ...

... and the fully advanced position:

The vacuum advance characteristics stamped into the upper part of the casing of typical 25D (left) and 45D (right) modules. On the 45D it is the '7 15 8' that describe the characteristics. The long number below is the Lucas part number, the 'S' of 'Lucas' being just visible to the left of the characteristics. The first number is the vacuum at which vacuum advance starts to be applied. The second number is the vacuum as which maximum advance is being applied, and the third number is the maximum advance that can be applied in distributor degrees. This doubles when read at the crankshaft markings.

A 25D points plate, showing how the vacuum module actuator pulls it round clockwise to advance timing:

And a 45D points plate ditto. Because the distributor is rotating anti-clockwise, moving the points plate clockwise causes the points cam follower to meet the lobe of the cam earlier to advance timing.