The dark green would have replaced the silver and further research seems to confirm it was a one-off. It was not dark green previously, and no other “A” was. Thus, no “A” was dark green in the fifties - they were all silver (or more to the point, coated in a greyish filth!). They, the Cs and the B101s were all distinctly UN-photogenic then!
All three classes got the light green from about 1959ish.
So, if accuracy is wanted, if it’s dark green, it can only be A42, and it can only be 1963-9, and it can only be with a waist level line. Light green locos variously did, and did not have, lining.