The Pentax K1000 35mm SLR was launched 1976, the same year that saw Canon release the groundbreaking AE-1. While the Canon AE-1 bought an array of technical advances, the K1000 was very much a fully mechanical SLR.
Incredibly it was still in production in 1997, though was now being made in China as opposed to Japan (while it was also made in Hong Kong for a time as well). Over its lifetime Pentax managed to shift millions of K1000s, and while its fair to say it didn’t have any standout features, it’s popularity can be attributed to the fact that it was simple and easy to use, affordable and there was a wide range of lenses readily available for it.
Source: https://www.digitalcameraworld.com/uk/reviews/pentax-k1000-review