Even if you go and look at the L23 from the imperial war museums, it states L23A1 on the side.
https://www.iwm.org.uk/collections/item/object/30021556
The L23 round, was first introduced on teh CR1 and Chieftain mk10 kits, which were long, after the Shir 2 was cancelled. hell it went into production in 1983 the same year as the challanger 1 there was no early L23 model as far as I’ve seen, adn ive been through about as many declasified sources as possible xD
It fire L15A5 as its AP round which featured an enhanced (apparently) tungsten alloy or DU, the chieftain, as per the tank museums tank chat on it, was capable in the 1970s of putting a round through the UFP of a T64 at about 2000 meters, however the T64 could do the same at over 2400 meters.
Hence the need for a new tank.
So L15A5 would and was the round that could go through an UFP of a T64. in game L15A5 is identical to the L15A3 which was from the 60s
EDIT: spelling error