The 28 through early 29 cars had what is known as a DRUM TAILLIGHT (Cylinder shape instead of "Teacup Shape" like the one mounted on your fender). They were mounted to a forged steel bracket that came of the body rather than off the fender.
Chances are a former owner of your car opted to use the later stye taillamp and bracket off the fender either because of better availability or cheaper price. Today, the drum taillights and forged brackets are readily available in reproduction (and super high quality) and can be had for both left side and right site usage.
If you're not looking to redo & make your car more authentic, stick with what you have. If the 3 empty holes bother you, they can be filled with standard hardware store bolts. (Sorry, I don't remember exact size)
