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