I think that it is possible that there is lots of stuff without fleet numbers on. This could be because it was a replacement or stolen etc. Where might you look for a number on this? As to the colour - if this was issued as new it would be 5,6,7 years old by the war, why not be a different blue - so, if not GUCC who? What ever it is it is old and original - original what I have no idea.
As to fleet numbers - did Buckby cans have numbers, chimneys, pigeons boxes, funnels? I do have Masthead lights some with some without...