As far as the original RTX-44, the HG-UC Guntank kit manual offers this explanation of its relationship to the Guntank. (The same info is repeated almost verbatim in the recent Master Grade kit manual.)
The Guntank was a long-range support mobile suit developed by the Federation Forces in U.C. 0079, along with the Gundam and Guncannon, as part of Operation V. Development itself essentially began with a next-generation MBT (main battle tank) development project that was under way before the RX Project, but with the implementation of the RX Project, this department was incorporated into that project. Its basic specifications were changed to "a machine capable of handling anti-mobile suit combat," and four units were completed as the RTX-44, the first mobile suit of the Federation Forces. With the RX-75, this machine was converted into a testbed for mobile suit drive systems, as part of Operation V. The machine was redesigned to incorporate a thermonuclear reactor and a Core Block system.
By this account, the main difference between the RTX-44 and the Guntank was the addition of a semi-functional Core Block. The RTX-44 itself was already close enough to humanoid form to be considered the Federation's "first mobile suit," so it probably didn't differ much from the layout of the Guntank.
Timing-wise, we're told that the development of the RTX-44 began before the launch of the RX Project (i.e. March of U.C. 0078). Its adaptation into the Guntank was part of Operation V, which we're told was proposed on February 17, U.C. 0079, and formally launched on April 1. Although the MSV books originally put the completion of the RTX-44 at March 20, U.C. 0079, the recent Master Archive book (discussed here
) claims that the RTX-44 was actually completed in March of the previous year, just as the RX Project was launched, and uses the March 20, U.C. 0079 date for the Guntank instead. According to the Master Archive, the Federation completes the RXM-1 full-scale test type vehicle one month later, and then rolls out the Guncannon two months after that. I have to admit that, if we're going from the RTX-44 to the RX-75 to the RXM-1 to the RX-77 as per the MSV books, the Master Archive timeline is probably more rational.
What about the RTX-440? As you'd suspect, this was created by upgrading the leftover RTX-44 units that weren't
used as RX-75 prototypes. The Master Grade Guntank kit manual says that the changes included "improved mobility thanks to the adoption of transformation mechanisms and improvements to the engine and armor materials." Specifically, according to the "MS Igloo Mission Complete" book, the sliding chest and forward "sub-crawlers" were added to the RTX-440 version for the sake of higher mobility.
The HG-UC Guntank kit manual also claims that the original RTX-44 was a two-seater:
The head of the RTX-44 was originally designed as a "gun turret" independent from the control cockpit. But due to a shortage of pilots, it was modified by the White Base crew so that all operations, including machine control and fire control, could be performed from the head.
The Master Grade Guntank kit manual repeats this claim, adding that the RTX-440 was redesigned so that all these functions could be performed from the head, and suggesting that this was the inspiration for the similar change that was later made to the Guntank.