Was the co-acting signal there before 1959?
I had a look at the relevant yellow peril last night, which seems to give the impression the co-acting signal was installed from new at that time. It doesn’t give any reason for its existence though...
There certainly wasn’t any co-acting signal provided at Finchley Central nor Epping - however these both date from different times (1940s versus late 1950s), and neither are a multi-aspect area like the Met.
Co-acting signals of this configuration were first installed at Chalfont (JT25 & JT25 Co-Acting) and Chesham (JV17 & JV17 Co-Acting) in April 1949. This was in connection with the continuous track circuiting of the Chesham branch, which allowed the existing electric train tablet installation to be taken out of use. At the time these were the only colour light signals in the Chalfont and Chesham areas (aside from the Chalfont up single line distant and a repeater for JV17 (RJV17) which were both installed at the same time).
When the Chalfont area was resignalled ten years later in 1959 JT25 & JT25 Co-Acting were removed and replaced by the current pair of signals (JW83 & JW83 Co-Acting) approximately 500-600 feet further from Chalfont. When control of the Chalfont area was transferred to Amersham Cabin in 1960 these two signals were renumbered to JT83 & JT83 Co-Acting.
In 1970, when Chesham signal cabin was closed, JV17 & JV17 Co-Acting were renumbered to JT69
B & JT69
B Co-Acting, and these signals were only removed when the track circuits on the Chesham branch were replaced by an Axle Counter in the early 1990s.