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1st May 1975
Class 121 W55025 is seen in Bodmin Road alongside Western 1013 working 1A09, the 1610 Penzance - Paddington service which was running five minutes late on 1 May 1975. Despite Ferndale being on the destination blind, the DMU would be on 2B22, the 1640 Plymouth - St Austell service. Roger Geach.

1981
Class 121 DTS L285 / W56285 and partner in Windsor and Eton Central in early 1981. Stuart Mackay Collection.

26th April 1975
Class 121 W55035 on Toller station on the Bridport Branch on 26 April 1975.
This is possibly a duplicate slide - any issues please let me know. Stuart Mackay Collection.

September 1975
September 1975 at Twyford station with a Class 121 which, according to my notes, will form the 19.14 to Henley. John Clark.

circa July 1984
Class 121 set L127 in Bedwyn station. The slide process date was July 1984. Stuart Mackay Collection.
Summary
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Numbering & Driving Inst.
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Details about the preserved Class 121 vehicles can be found here.