Junghans mentions spring #1007
450 x 2,25 x 0,205 mm
F.W.S 1942 page 17
Junghans catalog 1923 extract:
Junghans "Indestructible" Watch, Movement No. J31 with 3 jewels, compensating hair spring, club teeth escape wheel, double safety roller, gilt plates, well regulated.
These watches can also be supplied with Radium luminous figures (RZ)
The J31 is an advancement of the J26 movement. My only proof is the 1923 catalog and that it is not mentioned anymore in the 1927 catalog.
The J31 has the price group number 102 which is listed in the 1931 parts catalog under "Old Movements".
Last update 26.10.2018
Here are some hints how to separate some Junghans movements from themselfs.
Is it a J26 or a J31/J32?
As you can find J26, J31 and J32 movements without markings and as they look the same, there is this little hint, which can help you to determine if you have a J26 in hand or a J31/J32, without taking the watch apart.
Use a caliper and messuare carefully the distance between the top of the top plate and the top of the bottom plate. If more than 4 mm it is a J26.
Another feature which distinguishes the J26 from a J31 and a J31 from a J32 are the amount of screws on the balance wheel.
J26 14 screws, J31 8 screws and the J32 has 16 screws.