Jenson Button took to the Circuit de Catalunya with a bulbous lump on the nose of his MP4-26, connecting rods to the extremities of the front wing.
It is believed the setup is a tool to accurately measure front wing 'flex', after McLaren was out-foxed in this area by – notably – Red Bull and Ferrari last year.
Meanwhile, a McLaren spokesman confirmed reports the British team will not be running in Spain on Thursday.
McLaren's schedule will therefore effectively be two back-to-back tests on Tuesday-Wednesday and Friday-Saturday.
It also emerged in Barcelona on Tuesday that Nick Heidfeld, replaced by his Renault teammate Vitaly Petrov, will sit out the afternoon session on the opening day with a cold.
Albeit two seconds slower than Mark Webber, the German finished the morning session with the second best time.