BMX was probably born somewhere in California, sometime in the 1960s. But it only became a craze that swept the world in the late 1970s and early 1980s. It is hard to credit it, but BMX was literally everywhere at the time. The mainstream bicycle industry was left stunned and confused.
Both SunTour and Shimano responded by releasing very successful ranges of luridly coloured BMX hubs, BMX pedals, BMX headsets etc. etc.. But both the Japanese mega-brands also released rather less successful,BMX derailleur systems.
A 'BMX derailleur system' is a slightly contradictory concept. The essence of a BMX bike is bombproof simplicity. Derailleur systems are always just slightly fragile and complex. But the notion that the Japanese engineers alighted on was that, for a very few seconds at the very start of a BMX race, the rider could benefit from a lower gear. Once up to speed, the rider would move into the normal gear and batter around the course as per normal. So the design brief called for an extremely simple, robust two-speed system.
Shimano designed two parallel options. The first was a rear derailleur system combined with a two-sprocket freehub cluster. The second was a double chainwheel and a rock-crushing front derailleur. Theoretically these two systems could be combined to give a four speed BMX transmission - but I can't remember ever seeing this. Instead I always considered them as two alternative solutions to the same problem. Shimano quickly seemed to form a preference for the double chainwheel option. But the notion did not survive exposure to reality, within a very few years Shimano decided that any kind of BMX derailleur system was madness, and both options disappeared forever.
Shimano produced two very similar, but slightly different, versions of their BMX rear derailleur. The higher-end of these two was the Shimano SX (MX10). This had a 'main arm' that was a substantial piece of well chromed steel. It mounted outside the dropout and was clearly and and prominently branded. The Shimano SX (MX10) was widely marketed world wide.
This example is the lower-end version. It had a slighter, zinc-plated, 'main arm' that mounted inside the dropout. I believe that this version was fitted as original equipment on some BMX bikes made in Japan. It appears in no marketing material that I have ever found. I have been told that its part number is MX05, but, as it appears in no literature, I have no concrete evidence for this.