My understanding is that the first 40 will be the rilfes with the "gen1" fluted barrels and "gen1" gasblocks with every mk2 afterwards using the mid profile barrel and updated gasblock with a future mk3 down the line featuring the adjustable gasblock, dustcover and other such tweaks.
I have my eye on the fluted pencil be >weight but if I miss that boat I'll take the mid profile its only a 4oz difference.