What a find!!! I’m sitting here with the “Oldsmobile 4-4-2 & W-machines Restoration Guide” by T. Patrick Sullivan which we have in Hagerty’s reference section and he notes a few items to look for on the W-31 cars. Here are the bullet points verbatim from the book.
- Special 350 engine. Engine color is gold.
- Same forced-air system as 1968 W-30and Hurst/Olds.
- Special four-barrel carburetor.
- Number 5 cast on front of left cylinder head and rear of right cylinder head (same as standard 350 except W-31 has larger 455 valves)
- Manual brakes only.
- The letter M (for Lansing, Michigan) should be in VIN.
- All Ram-Rod 350s (W-31) are standard transmission - three or fours-speed.
- A Small, crossed piston decal with the word ram on one piston and rod on the other (350 on the bottom) appears just behind the marker lights on the front fenders.
- Heavy duty radiator (four-row).
- A special high speed harmonic balancer is used. This balancer is much thicker and heaver than the standard L-74 350 balancer.
- Must have black plastic fenderwells with holes for forced-air scoops.
- The 308deg camshaft means that the W-31 has a much rougher idle than the standard L-74 350.
Some other items I picked up on were that a 3.91 (mandatory) or 4.33 (optional) gear ratio were the only ones available.
As I am sure you are already aware, the W-31 was only available for the F-85, Cutlass and Cutlass Supreme. From the evidence you’ve already provided, I would say that evidence is very strong that this is a legitimate W-31. The book does not mention this being a coupe only option, so I am not ruling it out the possibility. As a pretty big Oldsmobile fan myself, I’m very excited for your find!