This signal in the forecourt of the GWR Museum (see Museum Home Page).
dates from c.1930 and is of 'lower quadrant' type (meaning the arm drops at approx. 60deg. to show 'clear').
It was in use until recently on the former British Rail Western Region.
Signals could either be 'starter' - ie. last signal in section - once train passes it starts
on the next section - or 'home' - signal within section guarding the approach to a station or junction.