|My own surface mount version of the Pixie2|
The idea of using the power amplifier transistor as a mixer seems to come from George Burt - GM3OXX - whose five transistor FOXX was described in 1983 in SPRAT. The basic design of the oscillator, PA/mixer and the simple keying has been more or less unchanged since Oleg Borodin - RV3GM - described the four transistor Micro-80 in 1992 in SPRAT. Then Dave Joseph - WA6BOY - replaced two of those transistors with the LM386 audio amplifier in the Pixie 2 (QRPp 1995). Most later versions are variants of these designs.
Here's the table of Foxx, Micro 80, and Pixie 2 kits: