Since I haven't been able to find any indication of the date, I have to assume that it's contemporary, But w/all that was going on in England at that time, everyone's casual attitude, when there aren't thoughts about murder, ruin the credibility. Going to Paris to conduct a financial transaction seemed downright silly, for 1941.
While i don't disagree with you that there were significant differences in the three decades that preceded the release of this film, why do you think the decade it which Suspicion was set is significant to the film?
Well, look at Grant. At one point he has the posture of a money-grubbing murderer. He seems like an upper class twit, who's currently broke, marries a rich girl, whom he discovers is not that rich, buys a house he can't afford, bets on horses, works for his cousin, embezzles money from him and eventually tries to go into business w/his friend.
Now in the 30s, money was scarce, so was work and most who lived in the industrialized world's confidence in business ventures had jumped off a cliff. This would make him stick out like a sore thumb. If I were a shrink, I'd diagnose him as manic.
The 20s was the opposite. He'd fit right in.
The teens had 3-phases that would all put very different backgrounds to the movie: before the war, during the war and after the war. (Although, after is pretty much the 1920s)
Finally, we can't rule out the possibility that the movie was contemporary. And just because it was released in 1941, doesn't mean it was written with a 1941 mentality. Hmm... I see it was released towards the end of 1941, but who knows when it was conceived, written and final changes were made to the above? The psychology of the English went through some drastic changes between the summer of 1940 and the following one.
Well, I think on the surface level there's nothing to indicate the time was supposed to be before the period of the film's release. If you are suggesting that the film was set in the twenties, there are no overt visual references to that period. No twenties hairstyles.
As for sticking out, he did stick out. He didn't fit in.