When did they come in , was it late 60s or early 70s ?
Around 1968 when plating and testing came in at a guess. 
Pete.
I can tell you exactly, from November 1st 1971 all Trailers and HGV’s had to have reflective rear marker boards fitted, this was stated in The Motor Vehicles (Rear Markings) Regulations 1970. There were five different types for certain lengths and practicable fitment. The long board that was red and yellow chevron stripes only and two shorter boards of the same design being fitted horizontally were for vehicles not exceeding 13m long or combinations not exceeding 11m long, if the fitting of these types was impracticable two similar boards mounted vertically each side of the rear could be used. Trailers in combinations more than 11m but not exceeding 13m could have the first types mentioned fitted or those of the same size with the words LONG VEHICLE on a yellow background with a red border. Vehicles over 13m or Trailers of combinations over 13m must have the last two Long Vehicle boards fitted only. These boards could be fitted before the statutory date if desired. Previous to this regulation many companies had red and white stripes painted onto the trailer rears although not a requirement they did give some warning of the type of vehicle in front of following traffic. I recall being in the Workshop at this period fitting many of these boards and from then after as they often needed replacing for annual tests, and how often have we seen these boards fitted incorrectly! Franky.
Type 1 long chevron board, Type 2 and 3 two short chevron boards fitted vertical or horizontal, Type 4 long LONG VEHICLE board, Type 5 two short LONG VEHICLE boards.
They had to have a British Standard mark on them as well, IIRC it was BS au 152a . If that was not there then the vehicle or trailer would fail its MOT. Back then all testing was done at Ministry test stations which all had a device for imposing weight on the rear axle. Four wheeler tippers were much more popular then so we used to take the tailgate off to allow the load simulator to reach into the back of the body. But you still had to have marker boards fitted, so these would be attached to the opening with fencing wire, and wo betide you if you had them on the wrong way.
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The BS marking was easily erased, especially when carting tarmac, and an otherwise perfect sign was then of no legal use. However we made them into mirror guards! 
We had to take our tippers loaded for test, usually 15 tonnes on an eight wheeler, which was ‘fun’ getting the Foden transmission handbrake to hold on the test ramp! 
Pete.
The old Long Vehicle signs were often used, with a bit of grease, to slide off rear hub and wheels when doing brake servicing or even patching up here and there, especially holed rear wings! Franky.
They were handy for making quick side guard return ends when found missing, on the trailer/vehicle that will be back in time for its last minute test.
When MFS ran drawbar combinations the reflective rear markers were reversible,“Long Vehicle” when running as a drag combination and the normal ones when running as a 24ft trailer and tractor.I do recall one of our Altrincham drivers being pulled for having “Long Vehicle” markers on his short artic,just got a warning off a seemingly knowledgeable officer…
It hasn’t made visibility of the back end any better now they are no longer a requirement, that bright yellow inside the red border fitted low down on the rear would be picked up brightly by even dipped headlights in murky conditions, i bet a lot of drivers of all sort of vehicle owe their continued good health to those long vehicle boards, the red reflective strip now used isn’t a patch.
Place i work still have proper boards specified on new tanks though without the wording, the workshop started to remove them at MOT time, i spoke to our TM soon as i got my trailer back, he had me go down the workshops and have the mountings and new full size reflective boards refitted, workshop now leave them in place an apply a new one if they’re getting a bit dull or scraped.
Like a lot of things, not everything new is an improvement.