[Michael Kenchington]

On 4th September 1990 Thames, revised its onscreen logo which was used on the idents, this was part of a wide scale changes which included the local news and the introduction of a new local ident, ( AKA the one that was used non stop after dumping ITV 1989 ident) 

I've come across the ident being used in march 1991 for local programmes, however it the exactly the same

This now private YT clip had the ident with slight different keys to the tune we all know.. 
www.youtube.com 

When it first appeared did it have a slightly key and then change rather quickly to the one we all know?   Or was the version we all know used form the very start in September 1990? the one in the private YT clip just a test case? 

Actually, Thames revised its onscreen logo on Monday 3rd September 1990 IIRC.