How do I find out where it was originaly registered in 74, as would be interesting

errr, not sure on that one. If you write the VW Milton Keynes or Wolfsburg, they will provide you (at a smallish cost) a 'birth' certificate for your car. The chassis/floorpan etc would have been built at Wolfsburg and then sent off to the Karmann factory to have a sexy convertible body added on to it!
Where it went to then will be very hard to trace though. It could have been exported to the US or sold in Germany. US spec ones are easy to tell from difference such as front valance (additional holes for US Spec bumpers), US spec lights etc, however, as yours has had a few late '80 customizing touches to it, I doubt these are the original ones!

According to the MOT testers manual

I took a Hillman Hunter to an MOT for a customer once (with a bag on my head). It didn't have a vin plate anywhere we could find it. The examiner said it was a fail, but I remembered reading in practical classics that vehicles first used before a certain date didn't have to have one. He gave me his copy of the manual while he carred on with the test, and I eventually found a reference to it.

http://www.motuk.co.uk/manual_630.htm