Right, when you have to go somewhere really, really fast, like Jupiter. But, with such a low mass-ratio, maybe not so fast. Probably stretching the Centaur would buy you more delta-V.
I ran across a description of a "Centaur Junior" from around the same period, which is in the Atlas Variants thread. It doesn't look quite the same, but the single-RL10 is there. It looked to be aimed at GEO and maybe lunar landing missions. Note that this was for an Atlas "F" variant that was never actually developed. The Atlas F that did fly was much different.https://forum.nasaspaceflight.com/index.php?topic=26915.msg1431549#msg1431549Interesting that this "Gamma Centaur" upper stage looks a lot like a Vega stage - a project canceled in December 1959. I also wonder about this "Gamma" vehicle. It looks like an Atlas, but it must have differed enough to warrant its own name. Different engines perhaps? Different propellants? - Ed Kyle
Here is the Atlas "F" with the Centaur Jr. from 1960.http://www.dtic.mil/docs/citations/AD0842594Regarding the H-2 engine proposed for Atlas "F", it would have been a 250 Klbf engine like the H-1c mentioned earlier. I believe that it was under study as a way to increase the Saturn stage thrust for future planned upgrades (later known as Saturn C-2, C-3, etc.). It was also proposed by GD for this giant Atlas "F" ICBM (see Pg. 45 of the document, note that this is not the Atlas F that was subsequently developed) that I suspect may have been offered as a Titan 2 alternative.I agree that this "Gamma" looks like an early Atlas Centaur booster stage. I've never until now heard of this "Gamma". It must have been a proposal to NASA, since Atlas-Centaur was a NASA program. NASA and USAF were using Greek letter designation about then (e.g. Thor-Delta, Thor-Epsilon), so Gamma might have fit that style. - Ed Kyle
The Link Document is dated 17 November 1965. While SDASM dated the Gamma Centaur picture from 08 July 1960.So there 5 years between those versions https://www.flickr.com/photos/sdasmarchives/24613662985/in/photolist-Dv2stK
Re the Centaur Jr. upper stage, in 1965 NASA's Future Programs Task Group produced a report (attached; see pp. 48 & 49 of the PDF) outlining the need for a 7000-lb-thrust lox/hydrogen stage to be used atop the Saturn IB/Centaur.