The LT designation at the rear is a throwback to heritage in the form of the McLaren F1 GTR ‘Longtail’. That race car from 1997 was part of the FIA GT Championship at that time and had undergone extensive modification on its engine, weight saving, and most notably, its bodywork, to generate much more aerodynamic downforce.
[UPDATE] Now with added engine noise:
Those characteristics are embodied into the road legal 675LT that joins the existing 650S and Asia-only 625C. The 675LT is part of the newly named McLaren Super Series which sits above the recently announced Sports Series in the range hierarchy. The supercar will only be offered in coupe bodystyle.
Power output is denoted in the name itself, 675 PS or 666 bhp, making the 675LT the most powerful Super Series offering. More details are expected to be announced leading up to the Geneva Motor Show.