Thanks to Pohill's research on THR, Colt patented a top strap design in 1850:
http://i37.tinypic.com/119sefc.jpg
Colt actually built an experimental revolver with a hinged top strap around 1850. It's #12 in the photo below along with other experimental models having a top strap:
http://www.flickr.com/photos/ctarchives/4522702535/