Andy,
You reflector question interested me. I looked on the current
www.ulo.de web site who are part of the odelo group. Their references only go back as far I can see to the early 70's Porsche cars. I have ULO reflectors on my 'A' but believe they should be glass and not plastic. One of the few things not original.
With the numbers you gave, I tried other sites and although not actually locating either ULO5132 or K11209 items came quite close with 1950's VW buses. You don't give the diameter of the lens is it 54mm or 56mm?
Porsche also used of course Hella as well as ULO and I did find a 4 page discussion on Samba concerning 56-57 reflectors. ( I realise this is too early for your car but its an interesting thread and the reflectors ( Hella ) look so much like yours. One guy entered info on a K11300 item so maybe its worth a look.
I just put: USA 50-57 reflector into google. When it opens the pages click on: USA 50-57 reflector discussion thread.
You might wonder if ULO in Germany just by chance kept a few items back of all their early products?? Might be worth an e-mail to them? You never know they might offer a lead.
Roy