Well, it would help to know which gun your model 512 is, but for now I will assume it is a Remington SportsMaster 512. The bolt stop may be worn, broken or dirty. Take it to a gunsmith for examination and repair as needed. Parts wear out.
I would go to Numrich Gun Parts Corp.first in my search for parts.
your Remington model 512 was made from 1940-1962,with a total production of 395,000 guns.
Lift the bolt, pull it back, and then depress the trigger (just don't make it too "sad" ! ) while pulling it out of the rifle.
remington model 512 value, as with all items, depends on condition. how good in the finish, wood, bore. Any rust/pitts. between $75 - $100 if good
I do not know the answer, I can tell you that I goy mine for Christmas 1959.
I think you mean a Sportmaster 512, Remington didn't make a Model 52. In MINT, like new condition around $300.-
Many shotguns & .22 caliber rifles manufactured before 1968 did not have serial numbers. This would include your Rem. 512
Best left to a professional. Find a gunsmith
Model Five Model 33 Model 34 Model 341 Model 37 Model 41 Model 411 Model 504 Model 510 Model 511 Model 512 Model 513 Model 514 Model 521 Model 540X Model 541 Model 580 Model 581 Model 581S Model 582 Model 591 Model 592 Nylon 10 Nylon 11 Nylon 12
It's date code stamped on the left side of the barrel in front of the receiver, check the Remington Society of America for location diagram and chart.
They were made from 1940-1962, to figure out the month/year you need to look at the barrel code and look that up under the manufactured date on the Remington Society of America.