It is documented on a schematic dated 20 July 1940.
Wikipedia:
Hallicrafters published that 50,000 SX-28 and SX-28As had been built by the end of its production run in 1946, however the serial numbers appear to indicate a production figure of half, approximately 27,500 receivers.
50,000 or 27,500, whatever it is, my sample size so far is 7. Take everything you see here with a grain of salt.
There's a really good page on this at Radio Boulevard Western Historic Radio Museum.
As I understand it, the traditional dividing line between SX-28 and SX-28A is the switch to the use of square coils in the RF section. Early radios had round, later square.
I've made a page showing the differences around the power cord and speaker terminals.
The radio was announced in July of 1940. There must be a schematic dating from late 1939 to early 1940 somewhere.
In the summer of 1940, Hallicrafters supplied a special version of the SX-28 receiver for the Federal Communication
Commission and its Radio Intelligence Division (RID). Parts were removed from the original design to make it more
suitable for the job. There is a single 6V6 output tube. Band switches SW8-1, SW8-14 & SW8-15 are missing.
It is documented on a schematic dated 20 July 1940.
A schematic dated 18 December 1940 exists.
Very early top to the RF cage. It is held in place with thumb screws. This is my #6 radio. It has only nine band switch wafers instead of the more common ten.
Very early radios have the RF Gain pot mounted to the main chassis. An extension shaft is used to get out to the knob.
A later version of the RF cage. Still held in place with thumb screws.
Early round coils in the RF modules.
The front plates can NOT be used as a good guide to the model. There must have been a lot of inventory of the old plates when they started using
the square coils because there are a lot of 28A versions not marked as such.
SX-28A top to the RF cage.
Late (SX-28A) square coils in the RF modules.
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