The 1938 Shop Manual shows the rocker arm cover to have 3 studs for attachment. The 1939 manual shows 2 studs. A page in the '39 manual details how the change was made midway through the '38 model year (see attached).
The Buick Master Parts book shows the change occurred at engine no 3525209. So your engine number (less the 4- series prefix digit) would have been built with 2 studs and a 2-hole cover.