I have now done a bit of "research" by flicking through the pages of Modern Railways (all issues between January 1968 and December 1975) and looking at the photographs and noting the dates they were taken - not an entirely unpleasant exercise
The results surprised me.
I may have missed something, but the first photograph I can see of "Inter-City" on any coach side is the shot of the very first Mk2d in 1971. Thereafter every shot of the air-conditioned versions (d,e & f) clearly has the Inter-City "hyphen" branding. No errors, sorry.
The first picture of a non-airconditioned Mk2 with "Inter-City" I can see is in the March 1974 edition and it is (photo undated) of the Scottish push pull service.
In a later edition I found a picture of a "branded" Mk2c at Shap behind a Class 50 on 23rd July 1973.
Photographs dated 1974 and later then include several shots of Mk2a (some already cascaded onto the ECML Grimsby service by then) Mk2b and Mk2c (on the WR and LMR) all
with the branding.
All the photographs taken before 1973 that are featured show non-airconditioned Mk2 without the branding. There is a wide variety of shots: Electrically hauled on the WCML, Deltics on the ECML, Westerns on the WR etc... Mk2, Mk2a, Mk2b and Mk2c all clearly without the branding in the late 60s and early 70s.
This is obviously unscientific research based on a limited number of photographs, but I am going to stick my neck out and suggest that it was only the Mk2d (possibly some later Mk2c but not all) that had the branding from new in 1971 and the rest were done afterwards. They were not done quickly either and there are some photographs from 1973-4 that show mixed rakes of branded-unbranded coaches.
That is my assumption - but I am happy to be corrected...
BELOW - Part of an undated photo taken by D A Flitcroft in the Dec 74 edition of Modern Railways, (fair use claimed) shows #1 and #4 & #5 vehicles unbranded, with #2 and #3 clearly branded. Train is approaching Preston hauled by a Class 87, so must date from after 23rd July 1973.
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