The guns marked with "Made in the 200th year of our liberty" are called "Liberty" models. Other than being made in 1976, there's really nothing special about them. Collectors will pay about $50 extra if the Liberty gun is like new and has the box and papers. For a shooter grade or without the box, most collectors are not too interested so they really don't bring much more than the same gun in a non-Liberty model, maybe 10-20 bucks.
Presently, Security-Sixes don't have much of a collector following. They are great guns but for some reason, just never caught on with collectors. There were 84,831 Liberty guns made in 1976 that were in the Security-Six family. This includes Speed-Sixes and Service-Sixes, so they aren't a scarce model by any means.
The only Security-Six revolvers that bring a premium are the snubbies. Probably the best concealed carry revolver ever made and very sought after.