As far as I know, it doesn't exist. Attack speed always goes by percentage.
Increasing the attack speed by 50... sounds a little weird. By the way, attack speed is determined by the unit's attack cooldown. So, assigning a value seems not appropriate.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.