It was the original, having an action loosly based on the M14, identical to the Ranch Rifle in all repsects, except lacking the the scope mounting facilities of the Ranch Rifle and having a more complex rear sight, rather than the leaf design of the early Ranch Rifle or the peep sight of the latest series. The scope mounting was a means of expanding the market for the rifle.
Google AC556 and you meet the select fire cousin that had the same, longer, action of the first series (180, I think). The added length was specifically to accomodate the select fire mechanism. With the 181 series, the select fire was history and the action was shortened about 3/4" for all subsequent models.