Patek Philippe

The pic is of a 3940 from the late 80s; it’s a solid case back and shows hallmarks only on left side of the case both sides of the push piece. Was this usual for the 3940s from that era? Thanks for the inputs ...  

This is where the early hallmarks we’re located.

but unless I'm mistaken, there should also be "PPCo" and "750" purity stamp on underside of the 5 and 7 o'clock lugs.

There were not hallmarks under the lugs for pieces this early in the series. There was only the two on the 9:00 side of the case. My rose gold 3940 is circa 1990 and on the left side of the corrector (closer to 10:00) is the 750 mark and on the other side... 

that seems odd, maybe on the inside then?? I've got one from the early 90's as described. Marks on the case side and both lugs.

Nothing on the case. Mine was a special order with see through case back only, so no case back to put it on. Looking at other early versions at least 1990 and earlier, this is consistent.

... and it was the 3941 that came w display case back.