7′ Silver Shadow playing area size question

I have a 7′ silver shadow made in 1974.  Measurements from inner rail edge to inner rail edge is 38.5″ X 78.5″.  The literature from IK shows the playing are as 39″ X 78″.   Does any one know how the play area is measured?  I’m sking because I recently got the table and it seems a bit smaller than the 7′ Valley I play one with my father at the bar.

Thanks