Womble, I agree you have one lamb that's scrawny there, and another that looks a better lamb.
First stop, read the Eblex literature. The one you want is 'Understanding lambs and carcases for better returns' on
this pageand if you can, go to a show where Eblex have a stand - they have dummy carcases you can practise on, and usually live and dead lambs too, and can help you get the hang of scoring your own live lambs.
For selling lambs in boxes you don't need to be as rigorous as you would if you were supplying supermarkets, but you still want to be not as fat as 4, and unless you have a tasty slow-grown primitive, ideally not less shape than R. (Which is not to say you can't get Shetlands that are R3L - people can and do.)
Don't just use condition scoring when testing readiness for slaughter, it's a tool we use all year round on all livestock. Thin pregnant ewes need more feed; fat mammas need slimming down before the tup; thin lambs mean worms or fluke or other problem, and/or mums not coping, etc.