Just picked this up

[Daan Kemp]

Says discarding sabot [DS] on the sabot, with the T indicating tracer.

The sabot provides obturation, grip on the rifling, spin, etc in the barrel. As soon as it leaves the muzzle the normal air resistance retards the loose sabot and it it falls away, leaving the penetrator to investigate the hardness of the target's armour. The tracer in the penetrator allows gunner to follow flight and correct the second shot.

Blue grey colour would indicate practice?

Trust this doesn't duplicate what you already know.