Google Remington scoremaster model 511. Several websites have information on this .22 firearm.
50-250 USD
The Remington model 510 Target Master was made from 1939-1962.
xx is 1951 but there is no month code y (blackpowderx) are the valid letters for month.
http://www.remingtonsociety.com (forums)
Look hereWhat_is_the_value_of_a_Remington_Model_742_in_good_condition
50-350 usd
current value of a 1941 Remington 510 22cal rifle
Your Remington model 510 Target Master will bring between 195-250 dollars in the condition that you describe.
What model by serial? No
Most remington model 742 woodmasters are selling in the gun shops for between 275-350 dollars.
There is no Remington Model 6 pistol, there has been however cases however of Remington Model 6 rifles converted to pistols by gun or "home" smiths. NOT Remington. As such the value is whatever someone will pay, but generally not much.
I think you mean a Sportmaster 512, Remington didn't make a Model 52. In MINT, like new condition around $300.-