So on October 18, 2003, the week after Episode 51 debuted in English, Ultimate Muscle just kept going on FoxBox with the worldwide debut of "Episode 52", as though these new episodes always existed; more than likely, Toei produced these episodes throughout 2003, shortly after finishing Kinnikuman II-Sei. However, this situation of 4Kids commissioning Toei produce more 26 more episodes of Ultimate Muscle is more like what happened to Beast King Go-Lion when more episodes of Voltron were commissioned in the 80s, instead of what happened to The Big O II earlier in 2003, i.e. these episodes were made specifically for English release first. However, unlike those bonus Voltron episodes, these new episodes of UM would eventually get aired in Japan, with (most of) the original Japanese voice cast returning to reprise their roles, though in this case it would technically be considered a "Japanese dub". So on April 8, 2004, just as UM was seven episodes away from finally ending on FoxBox, the first 13 episodes of these new episodes started airing on TV Tokyo weekly, now under the title Kinnikuman II-Sei: Ultimate Muscle. Then, on January 5, 2006 (yes, nearly two whole years later!), the remaining 13 episodes finally started airing on TV Tokyo under the title Kinnikuman II-Sei: Ultimate Muscle 2. By this point, Yudetamago had moved on to 2004-2011's Kinnikuman II-Sei: Kyukyoku no Chojin Tag-hen/Ultimate Chojin Tag Chapter, which saw the New Generation go back in time & fight against/team with the Legends in the prime & featured a new logo for the franchise, one that UM2 (& the later DVD release for these two "seasons") adopted; I honestly prefer the old logo, but the later one's still good. However, where the original Kinnikuman II-Sei anime aired in prime time, these two new seasons aired in Japan as late-night anime (1:00 AM, in particular, both times around), and since neither got fansubbed back in the day some don't even realize that these even aired in Japan, thinking that the last 26 episodes of Ultimate Muscle were English-exclusive.
So, to finish off the 20th Anniversary retrospective of the original 2002 Kinnikuman II-Sei anime, let's jump ahead to 2004 & 2006 by taking a look at the Japanese version of these (4Kids-commissioned) final 26 episodes, and while they were aired separately in Japan, with a two-year gap in between, I'll just cover them together, since they tell a single overall story arc.