Those 2EPB photos must be early 60s as they have no yellow panel and the older style blinds I referred to
While trying to find an answer Google led me to this thread - scroll down to the long post by The Duke 71000.
https://digest.dapol.co.uk/forum/main-forum/diesel-aa/class-73-aa/7506-class-73-decorated-samples
"RED BLANKS were ONLY allowed to be used as a "tail marker", on stock that had a safe capacity to illuminate such blinds by Battery power for at least 24 hours continuously. (For EMU's this included all "1951" stock onwards. So certainly 4SUB and 5BEL units even when fitted with roll round blinds still had to carry a tail lamp.) Otherwise a tail lamp was required on the end of the train. Further RED BLINDS were NOT tolerated on ANY other Region (because no other region had made any alteration to its operational rules to permit such blinds). So even after the Southern Region had effectively re-claimed the Salisbury to Yeovil line. Trains found arriving at Exeter St. David's, using red blinds (Class 33's and TC stock most frequently). Found themselves "Impounded". The train was NOT allowed to move again until a red lamp was hung on the rear of the train, for the return journey at least as far as Yeovil....."
This confirms that the RI were concerned about battery life. The SUBs etc didn't have the capability. I've left the rest of the paragraph in for amusement, but it's interesting that it was ONLY a SR thing for a long time.
Bardic lamps would be coincidence.
I don't know when DMUs started to use inbuilt tail lights, was it only with 70s refurbishment?