Thanks all for the replies and info.
Considering the block has the F255X stamping, I'm thinking the engine block is the original Thriftmaster, and not a Blue Flame passenger car engine. As Achipmunk mentioned, a previous owner probably just swapped out the valve cover for a Blue Flame cover.
When I removed the valve cover for inspection last week, I noticed the rockers are adjustable, indicating solid lifters are installed... or at least solid lifters are meant to be installed with that head. Here is a picture of the rockers under the valve cover.
http://i109.photobucket.com/albums/n77/vjsoto/1955%20Chevy%20Truck/IMG_2610.jpgI also looked for rear mount tapped bosses and there were none.
So it looks like I got a good ole Thriftmaster.
Chenzo
