The waistline, both on these and the A / C classes, appears to have been completely arbitrary (like the dayglo orange-red on locos in the 1990s) with no relation to any other livery details.
I have seen pics of silver locos with red buffer beams too, but never a green one without at least some red on the buffer beam area.
The one thing I have never seen is a B101 model with numerals of exactly the right style and size…. especially on a loco that is otherwise plain silver, plain green or (mostly) plain black, inaccuracy in numerals or lettering sticks out like a sore thumb, unfortunately.
The initial pic, I should add, is possibly one of Cyril Fry’s. If not, it’s one of my dad’s.