"Rainbow Rainbow Shin Matsunaga"
I previously created a mixing version of the Shin Matsunaga's High Mobility Zaku II Late Model, which appeared in "Rainbow Rainbow Shin Matsunaga."
It's said to have originally been based on Kunio Okawara's MSV-R illustrations or Oda's Stream-based model.
Considering that Oda created it at the height of the Gunpla boom, roughly 40 years have passed since its creation, so I wonder if the appearance in "Rainbow Rainbow Shin Matsunaga" is what I had in mind...
Honestly, there are certainly many possibilities, but

Marlene, stop it! | ふっださん's Gunpla works