51855 Beaconsfield 22 June 1991

Under a threatening sky, the 16:51 Aylesbury to Marylebone Network SouthEast service arrives at Beaconsfield station on 22 June 1991, led by 51855. Although I obviously didn't notice it at the time, this appears to be something rather different to the usual four coach Class 115 set. Note the yellow ends visible on the second and third vehicles, indicating that they are also driving cars. Presumably this is some kind of temporary lash up. Anybody remember seeing this combination in 1991?