Looks very much like a Dawes cycles extension from the early 60's. The bars look to be the Dawes style of engraving. These were made by GB for Dawes and were fitted to the "Red Feather model" & others. The pattern type should be stamped under the tape and will probably be marked Dawes as well. Sun cycles also used the same extension with a different badge. Trying to cash in on the kudos of Cinelli stems.
Best wishes and be lucky. Michael Butler Huntingdon UK.