The Master Archive books are probably one of the best world-building materials in Gundam as they really flesh out mobile suit history and development. They released one for the RX-78-2 Gundam almost a decade ago but the stuff inside is still very relevant, especially with regards to the recently released HG RX-78-02 Gundam Rollout Color.
According to the book, three RX-78 Gundam units were initially developed, with each going through a couple of iterations to become the RX-78-1/2/2 Gundams we’re familiar with. For its first iteration RX-78-1 initially had a white and red color scheme and was dubbed the “Pre-Specifications” version. At first glance, it looks similar to the HG Gundam Rollout Color but upon closer inspection, there are notable differences.
While its head unit shares the same side-antennas as the model kit, the chest vents are colored a little different. The front waist armor and lower legs are also much different while the forearms and ankle armors are the same with the classic Prototype Gundam design. If you want to build this version, then you can either scratch build the different parts, or do some kit bashing.
If you choose the latter method, you’ll need the HG Gundam Rollout Color and another Premium Bandai Kit, the HG GM Night Seeker. You will need to bash the kits based on the image above to get a kit that looks very close to the one from the book. Besides the forearms and lower legs, the front waist armor needs some modification if you want it to look as exact as possible. But I can do with the slight difference and would go with these parts swap.
The designs seen here aren’t meant to replace those seen Gundam THE ORIGIN as the book was released years prior. But you could clearly see how they incorporated these designs to produce the mobile suit seen in that ending sequence. So if you don’t know what to build next, then here’s a worthy suggestion.