http://www.ebay.com/itm/321909835792Here is a set that has the following marking on both the base and the schematic label
N 4023 A6
The schematic also indicates the frequency that presumably the code '6' stands for: 20
Schematic number D-53940 Issue 3.
Basically, I think we agree that the idea is correct, that both the digit in the second position of the second group of the old designation format (34A3, 40xx) is a frequency designation, as well as the 2 digits of the second group of later format (41xx).
A single digit for the ringer frequency is in principle possible, if
(A) another indication is used for the ringing system, as each system only has five frequencies
(B) We use the availability of the ringers as listed in the 1930s catalogs. They only list NINE frequencies in addition to SL:
16.6, 20, 25, 30, 33.3, 42, 50, 54, 66.6 Hz
In this case we need to figure out the correct sequence for digits 1-9, as 0 appears to be SL.
(C) The 1940 catalog lists 10 frequencies for 'Loud Ringing Bells' and for the AS-80 C.B. ringer movements.
16.6, 25, 33.3, 50, 66.6 and 20, 30, 42, 54, 66
In this list 20 Hz is item no. 6.
We have no indication that the code MUST be a single digit for the old format.
The catalog specifies to order the standard set, and specify the frequency desired. That means they did not have a catalog code for sets with special ringer type. I suspect that having 10 frequencies does not violate the observed format, it could simply be that the 10th frequency was specified as A10, rather than a single digit, say A9. We just have not found an example, nor do we have examples of all the single digit types.