Ozzy, yup, it was on green locos (1960-62) that I was researching where I first noticed it, specifically LMR ones in the Crewe/Manchester area. It only seems to be applied to those on the LMR, either green or blue.
Re door clips, two clips for all class 40's (middle two clips), unless broken or removed in later days, four clips for LMR allocated locos (from new) to carry their diesel specific named train head boards (outer pair of clips), though the clips look the same, they are not, there's a subtle difference between the two types.
The same discussion was only aired a few weeks back on the Peak thread which have/had the same set up.