Sanko SR3 (1953? version) derailleur

The Sanko S series dominated the small, but influential, derailleur market in Japan in the 1950s. These were very conventional low-normal, pull-chain, derailleurs that were extremely derivative of contemporary French designs. However, as with all Sanko products, they were very well made and excellently finished - possibly surpassing the Gallic originals.

The S series was made up of 9 models that were christened following a simple set of naming conventions:

If the letter is a single 'S', then it is a short cage derailleur, with a short main arm and a rigid (or non-existant) b-pivot.
If the letters are 'SR', then it is a short cage derailleur, with a short main arm and a sprung b-pivot.
If the letters are 'ST', then it is a long cage derailleur, with a long main arm and a sprung b-pivot.

If the number is '1', then it is an axle mounting derailleur with a hanger plate with a, conventional, forward-facing slot.
If the number is '2', then it is an axle mounting derailleur with a hanger plate with a rear-facing slot.
If the number is '3', then it is a derailleur that mounts on a braze-on beneath the chainstay.

So, for example, an 'SR3' would be a short cage derailleur, with a short main arm, a sprung b-pivot and a mounting under the chainstay. You can see a full listing in Sanko's April 1964 advert in New Cycling.

All S series derailleurs shared the same sliding rod mechanism, although different rod lengths were used for 3, 4 and 5 speed variants. I believe that all S series models could also handle both 1/8" and 3/32" chain.

I believe that this is a, pristine, unused example of a Sanko SR3. It has the following features:

It has sprung b-pivot. The main arm is short, at 65mm long. The pulley cage, at 49mm, is also short. All of which makes it an 'SR' model.
It mounts on a braze-on under the chainstay - making it a '3' model.
The tension pulley is steel and runs on a ball-bearings.