This is a slightly mysterious gear. The top sprung pivot is very reminiscent of Simplex, but the pulley cage with off-set pivot is perhaps more Huret. As far as I am aware, neither French brand made short cage pull-chain derailleurs that mounted on the chain stay. The guide pulley is also much smaller than the tension pulley.
As is usual with Sanko the construction of this derailleur is impressive, with quality bearings in the pulleys, well finished pressings and excellent chrome.