To remove the magazine catch from a Remington Model 51 pistol, first field strip the gun and remove the slide. Locate the magazine catch and use a small punch or screwdriver to depress the spring-loaded detent. While holding the detent down, push the magazine catch out of its slot, and it should slide out easily.
Pistol
Remington's website has what you are looking for.
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.
There is a small level protruding from the bottom at the side of the magazine. Push that level back (or forward, I forget which) and the magazine comes straght out the bottom.
What Model 51 pistol or a rifle and what model?
A model 742 magazine
None
Click on the link below for photo Model 51
Try eBay or gunbroker.com
The magazine you're looking for is the same magazine as those used on the Model 7400 and Model 74. Look to Cabella's or try e-bay or Gunbroker.com.
Remington makes multiple rifles with detachable clips. The 742 and 760 both have detachable magazines. The Remington model 700 can be readily altered to a detachable magazine, parts are sold in Cabela's, and other suppliers.
www.ammoclip.com/R/remington_51.htm