There's a research project going on the Model 51 it's on the Remington Society of America. If you go there look under current research one of those guys can certainly help you or visit the forums on the site. I've never removed the catch, I hate taking those things apart they're a pain.
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
No, the Remington Model 715 magazine is not interchangeable with the Model 710. While both rifles are part of Remington's bolt-action series, they have different designs and specifications that prevent their magazines from being compatible. Always refer to the manufacturer's guidelines for specific parts compatibility.
Click on the link below for photo Model 51
Try eBay or gunbroker.com
Yes, Remington has manufactured pistols, notably producing the Remington Model 51 and the Remington R51 in more recent years. The Model 51, designed in the early 20th century, was a semi-automatic pistol known for its unique design and features. The R51, which was reintroduced in 2014, aimed to serve as a modern concealed-carry option. However, Remington is primarily renowned for its rifles and shotguns.
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.