HST stock is Mark 3. The first tranche of LHCS (up to the sleepers) is Mark 3A and the LHCS vehicles after that (mainly the later FOs and BFOs) is Mark 3B.
From an internal point of view, two ways to tell the difference is by looking at the Ceiling and Seats. The Mk3 A’s were built with the then standard lighting raft* with short tubes running laterally along the length of the coach, and IC70s with chunky fixed armrests in First Class.
The later built Mk3 B’s were built with an almost 158 / 442 style lighting raft running longitudinally along the length of the coach and IC80 seats (rubbish at reclining by the way!) with thinner exposed steel arm rests.
*when I say standard, it was certainly standard across the early BR Mk3 EMU fleet. Also, granted, the Mk3 B in my photo has the Greater Anglia modified set up. But it wasn’t changed that much compared to the standard class modifications.