You have a very nice and desirable Regulation Police Target model. I very recently acquired one myself, the one pictured with red background in Hondo44s post above. The serial number 517900 shipped in April of 1935. During my "research" of this gun, I also discovered that another .32 RP Target with serial number 517800 was shipped to Buenos Aires in 1931. (So you may or may not have at least one other one to find in your country...;)) As you can see, shipping dates can be far apart for close serial numbers, especially for guns having little demand. They could sit in the vault for a long time before being shipped.
Since DCWilson indicated previously that 3 other RP targets beginning with serial number 504xxx shipped in 1929, it is quite possible that your stocks are original to the gun, even without a penciled serial number. Your stocks, without medallions, were utilized throughout the 1920s. If your gun was shipped in 1929, the stocks could be original. The stocks on my gun also do not have a discernible serial number penciled in on the right panel. But I have no reason to believe they are not original to the gun.
Whether original or not, your gun is still very collectible and valuable. Congratulations on obtaining it. Here are a couple more pics along with a closeup of the front sight.
View attachment 868913
View attachment 868914
View attachment 868915