Shown below are two examples of the B-12000 Model B ignition coils used primarily from 1932-1934, and as later era service replacements.
These coils are 'SX' brand Ford script-in-oval style. The 'SX' logo is the mark of the Essex Wire and Cable Company, the supplier to Ford Motor Company. The Ford script on these coils is smaller and finer than that found on other Ford script Model A ignition coils.
Shown below are two 'SX' coils with Ford script-in-oval. Both have a black bakelite nipple with polarity markings at the terminals indicating positive '+' on vehicle RH side, and negative '-' on vehicle LH side.
The Ford script-in-oval coil on the left also has a faint date code of 'B6' or perhaps 'D6' or 'E6'.
As shown above, the Ford script-in-oval coil on the right has a date code of '10E', indicating a production date of October, 1949. This coil was a later era OEM service replacement sold through a dealership parts department.
There is belief by some researchers that the 'SX' coils were introduced in 1931 near the end of Model A Ford production, and that the Ford script-in-oval was just a characteristic of Essex as a supplier, and not necessarily a strict indication of introduction date, nor strictly a Model B usage. This is certainly a possibility.
It is also true that beginning in 1932, Ford Motor Company generally used the oval enclosure around the Ford script logo on most of their signage, literature, etc.
The illustration above is from the July 1932 Parts Price List, and the interesting thing to note is that the B-12000 coil part number is noted as being re-numbered from the Model A part number A-12000.
What this means is that Ford essentially treated the four cylinder ignition coil as a running change during transition from Model A to Model B production, and that there was no distinct technical or interchange difference between a Model A coil and a Model B coil, at least from an engineering design, release, and usage point of view.
All Model A parts which were carried-over unchanged into the Model B vehicle were re-numbered with a B- prefix replacing the A- prefix as a matter of Ford policy at that time.
Thus it is quite possible that the supplier Essex 'SX' was introduced in 1931 during Model A production, and that their design just happened to include the 'Ford script-in-oval'. This could have then carried into 1932 with just a part number prefix change, but with no physical change to the part.
Based on my experience however, all NOS B-12000 (and all Ford script-in-oval) coils that I have had or seen are all marked 'SX' for Essex. It appears to me that by 1932, Essex was the primary supplier of four cylinder coils to Ford. Ford script coils without the oval were supplied by Standard Products 'SP' during and after the Model B era as well.
It should also be noted that Essex did not produce Model B distributors, unlike Ford suppliers Autolite 'AL' and Standard Products 'SP'. Based on a review of my stock of about 35 used and NOS Model B distributor upper plates, I find no 'SP' plates with any date codes.
All of my Autolite 'AL' Model B distributor upper plates are date coded in the same manner as American Bosch Model A 'Circle A' coils. The earliest date code I find for an Autolite 'AL' Model B upper plate is 'L12', indicating December 1931. The newest code 'AL' plate I have is dated '4N'.
Another interesting point to note in the parts book illustration concerns the distributor bakelite body, part number B-12105. That part is not a re-number of the A-12105 Model A distributor body.
The reason is that the Model B distributor body was different from the Model A, and the Model A part was not forward-compatible in 1932. The Model A distributor body had a single location notch to index off the pin in the distributor casting, whereas the Model B pin was in a different location, and the B part number body has two notches so as to fit both Model A and B castings.
In 1932, Ford changed distributors to the automatic advancing design, and this required moving the pin and notch location 90 degrees on both the iron casting and Bakelite body.
All B-12105 distributor bodies have two notches, one for Model A usage, and the other for Model B usage, making it backwards-compatible. Thus the A-12105 distributor body with its single notch was discontinued in 1932, and the B-12105 was released for production and used for service for both Model A's and Model B's.