They may or may not have a "1" datecode after the imprint.
Some collectors speculate that the ones without a datecode are really pre-production models from 1940.
It seems as it Parker ran out of cap jewels first, and continued production with whatever was available.
On some pens, Parker also appears to have used silver for the jewels, as one does find "first year" pens with jewels that have that unmistaken yellowish silver patina or heavy tarnish.
The Parker "51" commonly referred to as a "First Year" pen is really a pen from late 1940 through 1941.They can be easily distinguished from later production by several unique characteristics.