After looking in the DP7 Guide in the description for Tigervision's Threshold, I noticed that it says:

Based on the 1981 Sega/Gremlin "Astro Blaster" coin-op.
To me it looks and plays much more like the 1980 Sega/Gremlin/Data East coin-op Astro Fighter. I always thought that Megamania was based on Astro Blaster (it plays a LOT more like it than Threshold does).