I have a factory assembled preban oly arms 20" being transferred to me.  I just found out it has A1 sites on it.  

Does this mean it will have an A1 stock too?

Thanks,

Stone

Not Neccisarily.  Bushmaster still sells rifle with A1 upper receivers but uses A2 front sights and A2 stocks.

I know Olympic used to sell A1 uppers on rifles, older ones had A1 stocks, newer ones had A2 stocks.  Their 'Plinker' model now comes with a cast A1 upper and an A2 stock.

Besides the fact the previous owner could have put anything they wanted on it...

Easiest way to tell (just in case he changed the butt plate) is to measure along the TOP of the stock.  THe A1 will be 9 and 7/8" from the end of the buttplate to the where the stock meets the receiver.

An A2 stock will be 10.5" along the same line.