Found this post on Land Rover Series One Club forum (Reply #247):-
No...this sort of extra reflector remains the one thing that makes the rear of any trailer distinctive.
At the time the post war Brockhouse trailers were made they might have had a single or pair of T shaped ones ...an arrangement of small round red reflectors against a white background. On a vintage display trailer you might get away with this. But I suspect that traffic police would be looking for the modern pair of regulation triangles if someone ran into the back of you.
I am sorry but I have to argue against that popular misconception. What you say is true only of trailers built on or after 1st July 1970.
Road vehicle lighting regulations say that any trailer needs two rear retro reflectors. That I do not dispute.
Reg 18 and Schedule 1 states all vehicle and trailers must have rear retro reflectors, without any exceptions.
Schedule 18 of the 1989 act and Reg 3 EEC directive 76/757 require that any trailer (other than a broken down motor vehicle being towed) manufactured on or after 1.7.70 must have a pair rear retro reflectors that show certain approval marks. (The reflectors can only have the approval mark if they are triangles, the size and shape being part of their design that allows them to carry that mark, ie they will a triangle Length of side 150 to 200mm etc or they cannot gain that mark).
Slightly different requirements apply to trailers manufactured between 1.7.70 and 1.10.89, to the requirements that apply to those manufactured after that date but that doesn't concern my trailer or an early Brockhouse. (Requirement relate to approval markings the retro reflectors must carry.)
There is a requirement for a pre 1.1.70 to be fitted with rear retro reflective makers (schedule 1) but there is no requirement as to the shape, size or any markings they must carry (schedule 18 5. (b) (iii))
The Roset trailer I am towing is 1953 hence pre 1.7.70, so the only requirement is that it has two rear retro reflectors, which it has. They are round, which is acceptable since there is no requirement for them to be triangular or to have any markings or for them to be in any particular position on a trailer of this date.
http://www.legislation.gov.uk/uksi/1989/1796/contents/made
http://www.lrsoc.com/forum/index.php?topic=11701.240