According to the January 1964 GM Parts Catalogue Number 205 this cap fits only the following cars. This leads me to believe that the filler neck that was sourced for the cars was also a GM product, and most likely the 1960 Corvair, since that was the most current model year part that the cap fits. You could tell for certain if you were to have a fill tube from each of the year models of the vehicles listed below.
1953 - 1957 Chevrolet All
1960 Corvair All
1953 - 1957 Pontiac EXC 22/71
When I checked the locking gas cap for the same applications, I find that the number for the 1956 locking cap is different than the number for the 1957 locking cap. One thing to be careful of is people on eBay claiming that a particular cap that they are selling will fit many more applications than the parts book indicates. I know that many times GM changed part numbers for the same part, however, I have never found this to be true when it comes to the accessory items, unless they made a change in the product. My guess is that the 1956 locking cap differs from the 1957 locking cap is that the 1956 had a hidden gas filler under the left tail lamp assembly which necessitated a different size cap.