I saw a 1977 Masa Slingshot leaflet on ebay (source), copied below.
It has a lot of differences from a modern tadpole, in particular what looks like a "spoiler" on the front.
Is this similar to a spoiler on a sports car that is intended to provide more downforce to the wheels at high speed? Would that be effective / beneficial on a trike? I have read that this design was intended for oval track racing, so I assume the riders would get up to a good speed, so maybe that makes sense. I also read that with its long wheelbase a lot more weight went onto the rear wheel than on today's trikes, which also support the idea. And the brake seems to be on the rear wheel as well which seems consistent with that impression of weight distribution.
On the other hand, is this device just intended for something else? It actually isn't very wide or large in comparison to the frame, you'd think for relatively low-speed spoiler more surface area would have helped and looks like it should have fit. Would extra downforce even be needed? Plus, a larger "spoiler" might have acted somewhat like a fairing to reduce drag. (Is that actually all it really is, just an early or inexpensive / lightweight fairing design?)
Looking around I can find other images of these Masa trikes but very few which have that attachment. The few examples I did find lacked any explanation, though a forum post did use the same term "spoiler". No examples of modern racing recumbents or trikes that I could find had anything like this.