Some Gull releases were manufactured and distributed by Pye Records (others by Decca). Pye introduced a special type of vinyl in the mid/late 1970’s that looked like normal black vinyl but was actually translucent red when held very close to a bright light. Here’s an example of GULP 1031 from 1979:
Here’s the updated Gull Records releases so far (below: click the sleeve to go to the relevant page for sleeve, label and matrix details). Quite a few more to collect and add to the Gull page. Follow the blog to get instant updates!