Shown below are several varieties of the so-called straight pole ignition coils used from 1929-1931.
Shown above are a pair of 'Circle A' logo coils, one with Ford script and one without. The 'Circle A' logo was a mark of the American Bosch Corporation, a primary supplier to Ford.
The later era Ford script 'Circle A' coil on the left has a plain unmarked black bakelite nipple. The earlier non-script 'Circle A' coil on the right has an unmarked maroon bakelite nipple, as well as a date code of '4K' from April 1930.
Shown above is the date code of '4K', April 1930, on the non-script 'Circle A' coil.
Shown above are a pair of Ford script 'SP' coils. The 'SP' logo was a mark of Standard Products, an original supplier to Ford.
I was not able to find a date code on either coil, but there are a couple of subtle differences. Note the weights of the Ford script and the 'SP' logos, as well as the vertical placement of the Ford scripts.
As shown above, the earlier 'SP' coil on the left has a plain unmarked maroon bakelite nipple. The later era 'SP' coil on the right has a black bakelite nipple which is also marked 'Made in USA' and marked 'Dist' on the (vehicle RH side) terminal.