Good gosh, I remember them. I had one that I built probably over 30 years ago. I don't recall if they were based on a TV series or cartoon though. I do have a book about Monogram and I will see what it includes, if it includes anything.
According to the book, there were 6 fictional Shogun Warriors that were made using molds acquired from Bandai. Monogram had just made Bandai their Japanese distributors. They were released in 1977 (Dragun/Mazinga/Raydeen) and 1978 (Raider/Gai King/Grandizer). I do not know if those names correspond to the real Bandai name or not.
I'm not sure which named one I had, but there was a small jet fighter that inserted into the head from the top. That's the one I had and I remember playing with that small jet.
Just from memory, but they had the look of those Japanese robot hero costumes from the old movies where the arms and legs looked like round stove pipes.
Based on a quick eBay search I just did, it seems like they were from the Mazinger Z series in Japan. Up for auction, I see Raider and Raydeen. Also in an original Bandai box they call Mazinger Z looks like the one I had.