IIRC, the 181 series was the start of the 'button' hold open.
Don't think Ruger would have waited until January of '78 to start making the 181 series, so it stands to reason
that yours was made sometime in '77.
I have not seen a serial table that shows serial no. for the last of the 180 series either, i.e. how high the numbers went.
JT