The typical "Letterman" painting is not a painting at all. Most of them were manufactured (using a silkscreen-like process) in the Far East during the 1970s and sold at JC Penney for a retail price of $150 to $400 dollars.
The original artist was typically a "company man" or woman and therefore not acknowledged. They were contract or salary employees whose work-product remained property of the company they worked for.
Their work was copied much like a computer graphics image is copied to create T-shirts using multi-colored passes and a silk-screen pattern.
The "signature" is typically blocked style with the lower (horizontal) portion of the L extended.
If you're a collector of "That 70's Show" paintings.. then these might have some nostaligic value. :) but otherwise, they are simply quality (cleanable) oil prints that are hard to find at a reasonable price today.
The canvas is usually medium to good quality and the frames are lower quality (soft) wood and almost always painted and sometimes trimmed with plastic/metallics as well.