It is A46, you're quite right; and like many another example of the A, C & B101 classes, the green waist line seemed to vary. Obviously, it isn't on the loco there, but equally obviously it was at another stage, as photos exist (which is what IRM correctly relied on). In 1958, it's been repainted from the awful plain silver (i.e. plain muck!), but the lining hasn't been added yet.
The application or the waistline in the 1958-62 period was a bit like the application of the "dayglo" patches on the front of locomotives of the A, B121 and B141/181 classes in the post-1987 era; some had them, others never did, and others again had them for a while only.
I suspect the example above, as you suggest, was very short-lived indeed.