Information of the posts and what you will see:
With a couple of exceptions, all the posts were of the same design, a small monitoring underground room, for three observers, an access shaft with a small alcove containing a chemical toilet.
The posts were grouped into small clusters of around four sites. They communicated with regional HQs by telephone, latterly using the BT Tele-Talk push-to-operate control panel. One post in a cluster was designated a master post, and this had a radio link in addition. These can be identified by the dome covering the aerial connection and the heavy earth strapping on the ventilation shaft.
Main Features
1. Access Shaft
Entrance to the post. The hatch was modified on posts that survived the 1968 closures. This can be seen by a reconstruction of the shaft top, and the fact that there are two padlock fitments as opposed to the original bar and single padlock. In 1989, a mere two years before closure, some posts were fitted with a new hatch called the Torlift. These can be recognised by new concrete on the top of the access shaft. Also to be found is a a ventilation shaft with the GZI mount on top.
2. GZI Mount
The Ground Zone Indicator, (GZI), was basically a pinhole camera that fitted onto a metal mount usually found on top of the access shaft ventilation shaft. Four pinholes were trained on the four cardinal points. A nuclear flash would burn into photographic paper in the GZI, allowing the observers to determine the direction and elevation of the blast.
3. FSM and BPI Pipes
The Fixed Survey Meter (FSM) was a probe operated from inside the post which detected the level of radiation outside. A conical cover was fixed to the top when in use. The FSM pipe about the size of a domestic drainpipe usually found with a cover bolted to the top.
The Bomb Power Indicator pipe, (BPI), can sometimes be found between the FSM mount and the ventilation shaft. The BPI was a set of baffles connected to a pressure meter in the monitoring room. This would give an indication of the power of any explosions from the force of the shockwave. The BPI pipes are often missing as they are quite fragile.
4. Ventilation Shaft
Another ventilation shaft can be found at the far end of the post. If the post is a designated master post, the mounting and cover for the radio aerial can be found here.
5. Other Features
Sometimes earlier above-ground posts are located nearby. Wartime posts were often brick built towers, but you can also sometimes see the post-war Orlits. These were pre-cast concrete buildings, An Orlit A was positioned on the ground, whilst the B version stood on four legs.
The above information is from Neil Wood's website that can be found here:
http://uk.geocities.com/royalobservercorps/index.htm