All the examples on this page are highlighted in crayon; yours will not be this visible. Pull those off (for many guns, like the Firestar, be careful not to move the safety as a plunger can become dislodged and fly away, lost forever when the grips are off) and look if you have no other luck.
At least one reader has a model PD with, apparently, the month and year of manufacture directly stamped on the frame.
Probably the same year though, so its a good guide.
This is not just accidentally under the stocks because it was stamped before assembly, but deliberately along the backstrap portion, very hidden from view unless searched for.
A document sent to me some time ago, and which I just now noticed, is the same sheet as the above table from the nice people at Ipar Guns. In 1995 serial numbers began to follow a coded system.
Every 28 years or so (there are inexplicable gaps) they start a new series, by adding a number to the end of the letter codes.
Though there was another one operating for a short time in the early 1920s, Eibar is the one and only Spanish government proof house for most of the 20th century.