a while back, there was a thread here trying to figure out what the earliest design and also usage of the term "class D" amplifier was. i can't remember the name of the thread, but i did find this article in 73 magazine from may 1964. i haven't read the whole article yet, but the amp described uses a 50khz switching frequency. another design which i saw from 1965 (1965 IEEE circuit cookbook) was much simpler (only 4 transistors and no transformers) and was called a "bang-bang" amplifier, so the term "class D", although it existed wasn't in common use, even by engineers. once i finish reading the article, i will probably post a pdf of the whole article. it looks as if this particular application was as an AM modulator in a transmitter. but driving a speaker is much simpler than driving a transmitter's PA stage, and the transformer may not even be neccessary.