The 77PLs were some sort of distributor exclusive. I'm not a Ruger expert but I do have a few and ended up in this discussion on the Ruger forum a few years back:

http://rugerforum.com/phpBB3/viewtopic.php?f=4&t=215243

This led to a subsequent discussion with the publisher of the Reference of Ruger Firearms.

If you have any Ruger-specific questions I recommend posting them on that site. There are some good guys over there.