There were four commonly used adjustable track stems used in Britain. Major taylor has a square forward extension section and a square clip which slides along it. The Middleton adjustable is similar but has a round forward extension and round clip. On both these stems the clip can be fitted either above or below the extension. The reach can be from 1" to 7" on these stems. The two most popular Continental steel stems are the Cinelli and Philippe which look virtually identical. Ron Kitching in the 70's also imported a Japanese steel extension but the name escapes me?
Best wishes and be lucky. Michael Butler Huntingdon UK.