Jeff,

there's a nice post about the evolvement of Caliber 11 thru 12 on breitling forums.
It cites a letter dated December 1st, 1972 from C. Monnier of Breitling SA

Differences between Caliber 11 and Caliber 12

DIFFERENCES BETWEEN CALS. 11, 11-C, 11-2 AND 12

CAL. 11

Original basic calibre, 19,800 train (nickel plated)

CAL. 11-C

As Cal.11 but we have changed the arrangement of the calendar.
Rapid calendar replacement by a slow calendar. For that, we have changed the
calendar wheel No.2556/1 and omitted the two wheels Nos.2543 and 2548. The
mainspring and escapement have been checked and rectified.

Cal. 11-2

This is Cal. 11-c altered to 21,600 train. For that we have
changed the mainspring:- MgS = 1200, Mgi = 1050, M - 3.5 = 860 at 8 turns of
winding. We have also changed the fourth wheel No.225 (72 teeth instead of 66),
The escape wheel No 705 (diam. of pinion 0.67 instead of 0.73) and the frequency
of the balance and hairspring
No. 721.

Cal .12

Same construction as Cal. 11-2 except that the brass parts are
gold plated instead of nickel plated. (Plates, bridges, chronograph plate,
etc.)

La Chaux-de-Fonds

December 1st 1972
G.L. Breitling S.A

G Monnier.

IMPROVEMENT OF PARTS FROM THE LAUNCHING
OF THE CHRONOMATIC TO THE PRESENT DAY.

MOVEMENT

1. The setting lever screw No.5443 which broke easily, has been strengthened.

2. The oscillating weight No.1143/1 axle has been strengthened.

3. The arms of the driving gear for ratchet wheel No.1482 have been omitted. It is now a plain wheel in beryllium.
4. Transformed the rapid calendar to slow calendar by changing the date indicator driving wheel No. 2556/1.
5. With the slow calendar, we have omitted the two setting wheels Nos. 2543 and 2548.

6. We have also omitted the transfer on the date indicator guard No.2535, since with this new date indicator, it is not necessary to locate the calendar wheel.

7. With the slow calendar, we have strengthened the date jumper spring No.2575 from 5gm to 8gm.(Force measured on a special dynamometer). By doing this we achieve a cleaner jump and the disc can no longer be misplaced by a shock.

8. The pivot of /the date indicator driving wheel\ No.2556/1 has been enlarged against the risk of breakage.
9. We have formed a location on the /dial rest\ No.145 to facilitate its positioning during assembly.
10. The casing clamps No.166 have been thickened for strength.
11. Mainsprings have been improved.
12. Nivarox 2 hairsprings have been replaced by Isoval 2 which give better results.

13. We use heavier balances, moment of inertia
20 instead of 16.
14. The banking pins are made of nickel silver without surface treatment to prevent the lever from sticking to the pins.

La Chaux-de-Fonds December 1st 1972

G.L. Breitling S.A

G Monnier.