The 418 was the gun James Bond carried in the early novels until M makes him give it up for a Walther.

The Beretta 950 Jetfire has been around since the early 1950s, too. I imagine if you are using a stated 20 years old gun then the Jetfire would be too new but the 418 would not be, nor would the 1934.

Size-wise I think I would go with the 418, its manufacture date certainly fits your plot, and the later Berettas would definitely be more advanced/modern in design.

Depending on how intricate your plot is and how the gun plays a part you might want to do some detailed examination of these guns in re their operation. I always like a story where the weapons are properly depicted and used properly, etc.