A "knock off" is a copy of something that is intentionally made to look like the orginal. It is not done with the intent to trick someone into believing it is an orginal but it is done to compliment the attractive styling of the orginal.
Example: Many motorcycles copy the style of a Harley Davidson motorcycle.. they are said to be Harley Davidson "knock offs".
"Giveaways" are the small details that are different from the orignal that tell you it is a copy or "knockoff".
The felt pads on the mufflers on your drums are large and white... Slingerland used smaller pads of a darker color with a shorter tensioning mechanism.... I least I believe that is the "giveaway" on that one.
The colored inlay strips on Slingerland bass drums were a 1/2 inch wide.. the strips on your hoops appear to be much wider.. again I believe that is the "giveaway" that was mentioned.
Still.. your drums are very nice drums... I think Satin Flames wraps are perhaps my favorite on 60's drums.